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HOME  UNIVERSITY  LIBRARY 
OF  MODERN  KNOWLEDGE 

No.  71 

Editor t : 

HERBERT    FISHER,  M.A.,  F.B.A. 
PROF.    GILBERT    MURRAY,    LiTT.D., 

LL.D.,  F.B.A. 

PROF.  J.  ARTHUR    THOMSON,  M.A. 
PROF.  WILLIAM    T.  BREWSTER,  M.A. 


THE  HOME  UNIVERSITY  LIBKAEY 

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SCIENCE 
Already  Published 

ANTHROPOLOGY By  R.  R.  MARETT 

AN    INTRODUCTION   TO 

SCIENCE By  J.  ARTHUR  THOMSON 

EVOLUTION By  J.  ARTHUR  THOMSON  and 

PATRICK  GEDDES 

THE  ANIMAL  WORLD By  F.  W.  GAMBLE 

INTRODUCTION     TO     MATHE- 
MATICS      By  A.  N.  WHITEHEAD 

ASTRONOMY By  A.  R.  HINKS 

PSYCHICAL  RESEARCH   .    .    .    .  By  W.  F.  BARRETT 
THE   EVOLUTION   OF   PLANTS  By  D.  H.  SCOTT 
CRIME   AND   INSANITY  .   .   .    .  By  C.  A.  MERCIER 
MATTER  AND  ENERGY  ....  By  F.  SODDY 

PSYCHOLOGY By  W.  MCDOUGALL 

PRINCIPLES    OF    PHYSIOLOGY  By  J.  G.  MCKENDRICK 
THE  MAKING  OF  THE  EARTH  By  J.  W.  GREGORY 

ELECTRICITY By  GISBERT  KAPP 

THE  HUMAN  BODY By  A.KKITH 

Future  Issues 

CHEMISTRY By  R.  MELDOLA 

THE  MINERAL  WORLD  ....  By  SIR  T.  H.  HOLLAND 


PLANT    LIFE 


BY 


J.  BRETLAND  FARMER 

M.A.,  D.Sc.,  F.R.S. 

PROFESSOR  OF  BOTANY  IN  THE  IMPERIAL  COLLEGE 
OF  SCIENCE  AND  TECHNOLOGY 
LONDON 


NEW   YORK 
HENRY   HOLT  AND    COMPANY 

LONDON 
WILLIAMS   AND    NORGATE 


BIOLOGY 

H 
6 


PREFACE 

I  HAVE  not  attempted  in  this  little  book  to 
cover  the  whole  ground  indicated  by  its  title. 
My  object  has  rather  been  to  try  to  place  before 
the  reader  a  few  of  the  salient  features  of  plant 
form  from  the  point  of  view  of  function.  In 
this  way,  as  I  think,  it  is  less  difficult  to  keep 
in  mind  the  general  nature  of  the  causes  which 
have  been  operative  in  bringing  about  the 
marvellous  beauty  and  adaptedness  of  form 
which  is  so  apparent  in  every  branch  of  the 
vegetable  kingdom. 

The  task  of  selection  has  not  proved  an 
easy  one,  and  nobody  can  be  more  fully  alive 
to  the  imperfections  of  treatment,  and  other 
sins  of  commission  and  omission,  than  I  am 
myself.  Some,  at  any  rate,  of  the  last-named 
defects  are  attributable  to  the  limitations 
of  space. 

I  have  deliberately  touched,  though  with 
enforced  brevity,  on  certain  of  the  more 
difficult  problems  which  are  even  now  con- 
fronting us,  and  I  have  endeavoured  to 
present  them  with  as  little  technicality  as 
possible,  but  whether  I  have  been  successful 
in  this  I  must  leave  to  the  judgment  of 
others. 

V 

268794 


va  PREFACE 

It  is  not  impossible  that  some  readers  will 
expect  to  find  much  that  is  absent  from  this 
little  book — but  it  seemed  better  to  utilise 
the  space  at  my  disposal  by  devoting  it  to 
matter  not  so  generally  discussed  in  a  volume 
of  this  size,  than  to  attempt  to  say  again  what 
has  already  been  well  done  in  other  works  of 
this  scope. 

In  conclusion  I  wish  to  express  my  thanks 
to  Mr.  Tabor  and  to  Mr.  Trowbridge  for  the 
assistance  they  have  kindly  given  me  in  the 
preparation  of  nearly  half  the  illustrations. 

J.  B.  F. 

Gerrard's  Cro*s, 
Feb.  20,  1913. 


CONTENTS 

PAGE 

I      INTRODUCTION       .                      .....  9 

II      THE  PLANT  AND  ITS  FOOD  .  .  .  .  .13 

III  EVOLUTION  OF  CELLULAR  STRUCTURE   IN  SIMPLE 

PLANTS   .           . 28 

IV  THE  CELLS  AND  THE  ORGANISM  ....  40 

v    THE  "NON-CELLULAR"  TYPE  OF  ORGANISATION  49 

VI      THE  GREEN  LEAF 57 

VII      ROOTS  AND  THEIR  FUNCTIONS       ....  71 

VIII     CORRELATION   OF   FUNCTION  AND  FORM        .           .  86 

IX      MECHANICAL  PROBLEMS  AND   THEIR  SOLUTIONS   .  90 

X     SPECIAL    FEATURES    OF    CLIMBING     AND    WATER 

PLANTS 107 

XI     ADAPTATION 127 

XII     RELATION  OF  PLANTS  TO  WATER  .  .  .131 

XIII  THE  EPIPHYTES 149 

XIV  THE   FUNGI 161 

XV     FUNGAL  PARASITES 172 

XVI     FLOWERING  PARASITES 182 

XVII      COMPOUND   ORGANISMS 189 

XVIII     VEGETATIVE   REPRODUCTION           ....  204 

XIX      SEXUAL   REPRODUCTION 212 

XX     THE  CELL-NUCLEUS  AND  FERTILISATION        .          .  226 

BIBLIOGRAPHY       .......  251 

INDEX 253 

Vii 


LIST  OF  FIGURES 

rl°.  PAGE 

1.  CHLAMYDOMONAS  SP 16 

2.  APIOCYSTIS   BRAUNIANA 34 

3.  ULVA  LACTUCA,   THE  SEA-LETTUCE          ...  36 

4.  PRASIOLA   STIPITATA 38 

5.  HYDRURUS   FCETIDTJS        .        '.,  .  .  .  .41 

6.  CLADOPHORA  SP 45 

7.  CATTLERPA  STAHLII 50 

8.  HALIMEDA  OPUNTIA 52 

9.  SECTION  OF  A   LEAF 61 

10.  ROOT  IN  TRANSVERSE  SECTION       ....  74 

11.  DIAGRAMMATIC   SECTION   OF  YOUNG   WOODY   STEM  76 

12.  DIAGRAM  TO  SHOW  THE   BORDERED  PITS  AND   HOW 

THE   TRACHEIDS  IN   THE  WOOD   OF  A  PINE   ARE 

CONNECTED  WITH  EACH  OTHER  ...  78 
13A.  DIAGRAM  OF  TRANSVERSE  SECTION  OF  PART  OF  A 

YOUNG  STEM  OF  SUNFLOWER  ....  92 
13B.  DIAGRAM  OF  TRANSVERSE  SECTION  OF  PART  OF 

AN   OLD  STEM   OF  SUNFLOWER  ....  93 

14.  INDIAN  CORN 105 

15.  BAUHINIA  ANGUINA        .          .          .          .          .          .114 

16.  AN   OLD   BAUHINIA   STEM 116 

17.  STEM     OF    WATER     MILFOIL     (MYRIOPHYLLUM)     IN 

TRANSVERSE  SECTION 119 

18.  TAENIOPHYLLUM  TORRICELLENSE  ....  141 

19.  TRANSVERSE  SECTION  OF  AN  ORCHID  (DENDROBIUM)  153 

20.  TILLANDSIA   USNEOIDES 156 

21.  NEOTTIA   NIDUS-AVIS,    A   SAPROPHYTIC   ORCHID       .  191 

22.  ROOT  TUBERCLES  ON   THE  ROOTS  OF  THE  KIDNEY- 

BEAN  PLANT 195 

23A.  PELTIGERA  CANINA,  A  LICHEN  ....  201 
23B.  TRANSVERSE  SECTION  THROUGH  THE  THALLUS  OR 

FROND  OF  A  LICHEN  (PELTIGERA  CANINA)          .  202 

24.  CHLAMYDOMONAS  MEDIA 215 

25.  FERTILISATION  OF  HALIDRYS  SILIQUOSA          .          .  223 

26.  DIAGRAM   TO  EXPLAIN   THE  COURSE   OF  AN   ORDI- 

NARY  NUCLEAR   DIVISION   AND   THE    RELATION 

TO   IT  OF  THE   MEIOTIC  DIVISIONS      .          .           .  229 

27.  PROTHALLUS  OF  FERN 238 

28.  LONGITUDINAL  SECTION   THROUGH   THE    PISTIL  OF 

BUCKWHEAT 243 

viii 


PLANT  LIFE 

CHAPTER    I 

INTRODUCTION 

A  GENERAL  survey  of  the  Animal  and  Plant 
kingdoms  emphasises  in  the  clearest  manner 
the  cardinal  importance  of  the  great  functions 
of  nutrition  and  reproduction.  It  also  en- 
ables us  to  perceive  the  intimate  relation 
which  exists  between  the  full  discharge  of 
these  functions  and  the  evolution  of  the  higher 
from  the  lower  forms  of  life.  We  are  further 
led  to  conclude  that  there  is  no  great  gulf 
separating  the  animal  from  the  plant,  but 
that  the  similarities  which  exist  between  the 
two  great  classes  of  living  things  are  even  more 
striking  than  are  the  obvious  differences,  at 
any  rate  in  so  far  as  essentials  are  'concerned. 
Indeed,  the  differences  consist  in  features  which 
are,  after  all,  mainly  of  secondary  importance, 
and  they  are  largely  determined  by  the 
divergent  methods  of  obtaining  food  which 
characterise  the  animal  and  the  plant  respec- 
tively. 

Casting  our  glance  still  further  afield,  the 
boundary  line  between  the  organic  and  the 
9 


PLANT  LIFE 


inorganic  worlds  now  appears  less  sharply 
defined  than  formerly,  and  many  reactions 
which  used  to  be  regarded  as  immediately 
and  essentially  associated  with  life  and  active 
vitality  are  now  recognised  as  being  sus- 
ceptible of  a  less  mystical  interpretation. 
Thus  it  has  become  more  and  more  clearly 
apparent  during  the  last  few  years  how  great 
a  share  the  various  ferments  may  take  in 
promoting  those  reactions  which  formerly 
were  regarded  as  inseparable  from  the  living 
organism. 

Now  although  these  ferments,  in  the 
narrower  sense,  are  doubtless  the  products  of 
protoplasmic  activity,  they  can  initiate  and 
carry  out  their  specific  reactions  in  a  test- 
tube  under  conditions  which  are  incom- 
patible with  the  concurrence  of  life  in  the 
ordinary,  or  indeed  in  any  real,  sense  of  the 
word.  Evidence  is  accumulating  to  show 
that  the  ferments  owe  their  specific  activities 
to  their  physical  structure,  and  that  they 
approximate  to  the  singular  class  of  "  cata- 
lytic "  inorganic  bodies,  which,  like  them, 
are  able  to  promote  and  accelerate  certain 
chemical  changes  without  themselves  under- 
going destruction. 

Indeed,  as  time  goes  on,  exact  investigation 
is  continually  lifting  corners  of  the  curtain 
which  conceals  the  mysteries  of  life,  and  the 
glimpses  we  have  caught  tend  to  suggest  that 
although  the  reactions  which  are  going  on 
in  the  living  laboratory  are  (at  present) 


INTRODUCTION  11 

incalculably  more  obscure  than  those  with 
which  we  have,  in  a  measure,  become  familiar 
in  ordinary  chemical  and  physical  researches, 
nevertheless  they  are  similarly  influenced  by 
physical  conditions,  and  they  obey  the  same 
chemical  laws.  The  chief  differences  between 
the  reactions  in  the  living  body  and  those 
which  occur  outside  it  seem  to  lie  mainly  in 
the  greater  complexity  of  the  substances  con- 
cerned, as  well  as  in  the  necessity  for  accurate 
adjustment  of  the  reacting  substances  towards 
each  other  in  ways  which  we  can  at  present 
but  feebly  imitate.  An  important  feature  in 
this  matter  of  adjustment  consists  in  that 
state  of  aggregation  which  we  call  colloidal, 
which  is  so  characteristic  of  the  framework 
of  living  things,  and  by  virtue  of  which 
the  physical  conditions  for  many  chemical 
reactions  are  provided. 

A  simple  example  perhaps  may  serve  to 
make  the  point  clearer.  A  piece  of  platinum 
wire  will  not  bring  about  the  ignition  of  a 
mixture  of  coal  gas  and  air,  but  if  the  plati- 
num be  finely  divided,  e.  g.  in  the  form  known 
as  "  platinum  black  "  or  spongy  platinum,  it 
will  do  so.  If  the  platinum  be  still  further 
divided,  it  assumes  the  condition  known  as 
"  colloidal  platinum,"  and  it  is  then  capable 
of  promoting  many  other  chemical  changes  in 
a  manner  closely  resembling,  and  perhaps 
essentially  similar  to,  that  characteristic  of 
many  organic  ferments. 

Of  course  it  is  not  meant  to  suggest  that  the 


12  PLANT  LIFE 

complex  phenomena  of  life  are  at  once  reduci- 
ble to  terms  as  simple  as  those  which  regulate 
the  reactions  just  indicated,  and  it  is  beyond 
all  doubt  that  much  more  refined  investigation 
than  our  present  knowledge  renders  possible 
will  be  needed  ere  we  shall  solve  the  ultimate 
secrets  of  life,  if  indeed  we  ever  are  able  to  do 
so.  But  we  shall  go  far  by  employing  the 
methods  which  have  already  taught  us  so 
much,  methods  which  consist  in  exact  ex- 
periment and  accurate  analysis. 

The  principal  reason  why  our  knowledge 
of  the  modus  operandi  of  the  living  organism 
is  so  largely  lacking  in  precision  lies  just  in 
the  vast  range  of  the  materials  with  which 
we  are  there  dealing,  and  in  the  consequent 
difficulty  of  analysing  the  results  of  our  experi- 
ments sufficiently  to  be  able  to  refer  them 
to  their  real  causes. 

But  although  it  may  not  be  possible  as 
yet  to  explain  the  great  majority  of  the  life 
processes,  either  of  animals  or  of  plants,  it 
will  soon  be  apparent  that  relatively  simple 
chemical  and  physical  processes  have  pro- 
foundly modified  the  course  of  evolution  of 
structure  and  form.  This  is  more  obvious, 
perhaps,  in  plants  than  in  animals,  because 
the  retention  of  relatively  simple  mechanisms 
in  connection  with  the  absorption  of  food 
materials  has  kept  the  plant  free  from  the 
complications  introduced  by  the  development 
of  specialised  locomotory  activity,  and  the  con- 
comitant elaboration  of  a  nervous  system. 


THE  PLANT  AND  ITS  FOOD      13 


CHAPTER  II 

THE   PLANT  AND   ITS  FOOD 

ONE  of  the  most  striking  points  of  difference 
between  the  animals  and  the  plants  consists 
in  the  evident  and  purposive  motility  of  the 
former,  and  the  apparent  (but  only  apparent) 
immobility  of  the  latter.  Nearly  all  animals 
more  or  less  actively  seek  their  food,  and 
ingest  it  in  a  solid  form;  and  even  those 
species  which,  like  the  adult  oyster,  are 
tolerably  stationary,  nevertheless  exhibit  some 
sort  of  motion  by  which  currents  of  water, 
bearing  particles  of  food  are  drawn  into  their 
bodies. 

The  general  tendency  in  the  plant  kingdom, 
on  the  other  hand,  has  been  to  produce 
relatively  stationary  forms  which  do  not 
actively  pursue  their  food,  but  passively 
absorb  it  from  their  surroundings.  Many  of 
the  most  primitive  plants,  however,  share 
with  the  animals  a  faculty  of  vigorous  loco- 
motory  movement,  swimming  through  the 
water  in  which  they  live  by  means  of  vibratile 
filaments  or  cilia.  What  is  it,  then,  which 
has  caused  the  higher  members  of  the  one 
kingdom  to  abandon  this  locomotory  activ- 
ity while  those  of  the  other  have  not  only 


14  PLANT  LIFE 

preserved  it,  but  have  acquired  all  the  acces- 
sory complications  of  structure  that  purposive 
motion  necessarily  entails  ? 

The  answer  is  to  be  sought  in  the  results  of 
an  apparently  trivial  difference  in  structure 
between  the  animals  and  plants  which  made 
its  appearance  at  an  early  period  in  evolution- 
ary history.  It  is  a  difference  which  from 
the  start  was  fraught  with  consequences  of 
the  greatest  importance,  and  has  profoundly 
affected  the  entire  course  of  development 
in  the  two  kingdoms  respectively.  Stated 
simply,  it  consists  essentially  in  this,  namely, 
that  the  living  substance  of  the  plant  secretes 
over  its  surface  a  skin  of  cellulose,  or  some 
analogous  substance,  whilst  that  of  the 
animal  does  not.1 

If  we  examine  any  one  of  the  simplest 
microscopic  individuals  of  whose  vegetable 
'nature  there  can  be  no  dispute,  we  shall  find 
that  the  protoplasm,  or  living  substance,  is 
enclosed  in  a  not-living  skin  or  bag  of  cellulose. 
This  skin  is  not  an  indispensable  structure, 
for  the  living  substance  may,  for  a  time  at 
least,  exist  without  it.  Even  in  the  highest 
plants  this  commonly  occurs  during  the  first 
stages  of  embryonic  existence,  but  as  soon 
as  development  begins  the  membrane  is 

1  This  statement  is  broadly  true,  for  although  cellulose 
is  not  unknown  in  the  animal  kingdom  it  has  never  been 
so  arranged  in  the  body  as  to  affect  the  whole  relations 
of  the  animal  to  its  physical  environment  as  it  does  in 
plants. 


THE  PLANT  AND  ITS  FOOD      15 

secreted  over  the  surface  of  the  living  sub- 
stance, which  is  henceforth  shut  off  from  the 
outer  world  throughout  the  vegetative  life  of 
the  individual,  It  is  usually  in  connection 
with  certain  reproductive  processes  only,  as, 
for  example,  when  a  new  generation  is  about 
to  arise,  that  the  plant-protoplasm  is  more 
or  less  freed  from  the  cellulose  skin  with 
which  it  is  almost  invariably  clothed  at  other 
periods  of  its  existence. 

The  simplest  method  of  realising  what  all 
this  means  to  a  plant  is  to  study  some  definite 
example,  when  other  salient  features  of 
plant  life  will  also  come  directly  under  notice. 
For  this  purpose  one  of  the  common  lowly 
plants  belonging  to  the  algce  may  be  chosen, 
and  we  will  select  as  an  example  a  microscopic 
organism  belonging  to  the  genus  Chlamydo- 
monas. 

This  plant  is  of  fairly  common  occurrence 
in  ditches  and  pools,  especially  in  late  spring 
and  in  the  autumn.  Its  body  consists  of 
a  single  cell,  that  is  to  say  its  somewhat 
oval-shaped  protoplasm  is  contained  within 
a  single  membranous  cavity.  At  one  end 
two  vibratile  hair-like  filaments  of  protoplasm, 
called  cilia,  project  through  the  membrane, 
and  it  is  by  means  of  these  that  the  little 
plant  is  able  to  swim  actively  through  the 
water  in  which  it  lives.  Within  the  proto- 
plasm of  the  body,  and  just  beneath  the  spot 
where  the  cilia  sprout  from  it,  are  two  con- 
tractile vacuoles — hollow  spaces  filled  with 


Fig.  1. — CMamydomonas  sp.  C,  chloroplast  (horizontal 
shading);  P,  pyrenoid  (vertical  shading);  N,  nucleus; 
V,  contractile  vacuole;  Cil,  cilia;  C.W.,  cell  wall. 


16 


THE  PLANT  AND   ITS  FOOD      17 

liquid  which  are  continually  contracting  and 
expanding. '  This  kind  of  vacuole  is  also 
characteristic  of  many  of  the  primitive 
animals,  whilst  it  is  only  met  with  in  com- 
paratively few  of  the  more  primitive  and  still 
motile  plants.  A  small,  somewhat  refractive, 
spot  in  the  protoplasm  marks  the  position 
of  the  nucleus,  an  important  structure  which 
is  found  in  the  protoplasm  of  animals  and 
plants  alike.  Another  part  of  the  protoplasm 
is  coloured  green,  and  is  clearly  denned  from 
the  rest  of  the  living  substance  not  only  by 
its  colour,  but  by  its  denser  consistency.  It 
is  termed  the  chloroplast,  and  it  often  contains 
a  clear  spot  in  its  interior,  termed  the  pyrenoid. 
Finally,  close  to  the  point  of  insertion  of  the 
two  cilia,  there  is  a  small  brown  or  yellow 
"  eye-spot."  The  little  plant  swims  about 
through  the  water,  and  though  the  movements 
appear  at  first  sight  to  be  aimless,  they  are  not 
altogether  so,  for  if  a  large  number  of  in- 
dividuals are  present,  so  as  to  give  the  water 
a  green  tinge,  it  is  seen  that  they  congregate 
on  the  illuminated  side  of  the  vessel.  That 
is  to  say,  they  are  affected  by  the  stimulus  of 
light,  and  the  members  of  the  colony  spread 
themselves  out  towards  the  source  of  illumina- 
tion. In  other  words,  they  are  irritable, 
which  is  the  technical  way  of  expressing  the 
fact  that  they  are  capable  of  responding  by 
a  movement  to  a  stimulus — in  this  instance, 
to  the  stimulus  of  light. 

Under  suitable  conditions  of  temperature 
B 


18  PLANT  LIFE 

and  nutriment  the  Chlamydomonas  plant 
rapidly  multiplies  in  a  vegetative  or  non- 
sexual  manner.  This  is  brought  about  by 
the  division  of  the  protoplasmic  contents, 
within  the  membrane,  into  a  number  of 
smaller  lumps,  each  of  which  becomes  a  small 
image  of  the  original  parent.  They  are 
commonly  two,  four,  or  eight  in  number,  and 
finally  escape  from  the  ruptured  membrane  of 
the  "  mother  cell  "  into  the  surrounding  water 
where  they  grow,  and  may  give  rise  in  their 
turn  to  new  individuals. 

Now  such  a  plant  as  Chlamydomonas  is 
relatively  very  simple,  and  yet  it  already 
exhibits  the  most  striking  characters  that 
distinguish  the  majority  of  plants. 

In  the  first  place  its  living  substance  is 
enclosed  in  a  membrane,  and  in  the  second 
its  protoplasm  contains  a  green  chloroplast. 
In  order  to  grow  it  must  clearly  obtain  food, 
but  the  presence  of  a  membrane  precludes 
it  from  acquiring  any  except  such  as  is  already 
dissolved  in  the  water.  No  solid  particles 
can .  pass  the  membrane  and  so  reach  the 
protoplasm,  but  water  and  substances  dis- 
solved in  it  will  readily  do  so.  The  whole 
of  the  mineral  food  substances,  and  such 
gases  as  oxygen  and  carbon  dioxide,  reach 
the  protoplasm  in  this,  and  only  in  this  way. 

But  this  external  membrane  not  only 
limits  the  nature  of  the  food-supply,  but  as 
the  size  and  complexity  of  the  body  increase, 
it  continues  more  and  more  to  restrict  the  kind 


THE  PLANT  AND   ITS  FOOD      19 

of  movements  which  make  for  locomotion. 
Such  movements  indeed  soon  come  to  lose 
their  value,  even  in  water  plants,  when  the 
capacity  for  ingesting  solid  food  has  been  lost, 
whilst  they  would  tend  to  render  the  exist- 
ence of  a  land  flora  practically  impossible. 

We  may  consider  Chlamydomonas,  then,  as 
a  plant  belonging  to  a  class  the  members  of 
which  have  not  as  yet  diverged  far  enough 
along  the  plant  line  of  evolution  to  have 
lost  the  power  of  movement.  But  even 
amongst  the  near  relatives  of  the  species  under 
consideration  there  are  forms  which  pass  at 
least  a  part  of  their  vegetative  lives  in  the 
passive  and  non-motile  condition  character- 
istic of  more  advanced  members  of  the 
vegetable  kingdom.  A  familiar  example  is 
afforded  by  the  green  incrustation  everywhere 
to  be  seen  on  old  damp  palings.  This  in- 
crustation consists  of  countless  numbers  of 
minute  green  cells  known  as  Pleurococcus, 
which  grow  and  multiply  by  division.  The 
separate  individuals  are  habitually  destitute 
of  all  locomotory  mechanism,  and  each  grows 
and  multiplies  in  the  spot  where  it  happens 
to  have  become  fixed. 

The  Pleurococcus  plant  thrives  in  damp  air, 
and  it  depends  on  the  chance  supplies  of 
moisture  for  the  water  it  requires.  The  gases 
of  the  atmosphere,  passing  by  diffusion 
through  the  membrane  or  cell  wall,  are  dis- 
solved in  the  watery  sap  which  bathes  its 
living  protoplasmic  substance.  Thus  supplied 


20  PLANT  LIFE 

with  food  materials,  it  multiplies  rapidly. 
When  an  individual  has  reached  a  certain 
size  it  divides  into  two,  and  this  process  being 
repeated  in  the  various  individuals  of  a  colony 
the  Pleurococcus  spreads  rapidly  over  the 
surface  of  the  damp  wood.  Furthermore, 
the  individual  plants  withstand  desiccation 
without  dying,  and  in  this  condition  they  are 
carried  by  currents  of  air  to  fresh  spots  where 
new  colonies  can  be  started. 

But  to  return  to  Chlamydomonas.  Its 
second  feature  of  importance,  from  our  present 
point  of  view,  consists  in  its  greenness. 

As  we  have  seen,  the  green  colouring  matter, 
or  chlorophyll,  is  not  diffused  through  the  whole 
protoplasm,  but  is  restricted  to  one  or  more 
(in  this  plant,  one)  definite  and  specialised 
masses  or  corpuscles,  each  of  which  constitutes 
a  chloroplast.  The  part  which  the  chloroplast 
plays  in  the  cell *  is  that  of  utilising  the  energy 

1  CELL. — This  is  the  term  commonly  used  to  denote  the 
unit  of  a  living  organism,  though,  unfortunately,  it  is 
not  always  used  in  the  same  sense  by  different  writers. 
In  this  book  it  will  be  taken  to  denote  a  mass  of  protoplasm 
(whether  enclosed  in  a  cellulose  membrane  or  not)  which 
is  dominated  by  a  single  nucleus.  This  protoplasmic  mass 
is  commonly,  but  not  necessarily,  separated  by  a  cell 
membrane  from  other  similar  ones  in  the  cell -aggregates 
which  together  constitute  the  bodies  of  larger  plants. 
A  plant  may  thus  consist  of  (1)  a  single  cell;  (2)  a  number 
of  coherent  cells,  each  more  or  less  delimited  from  the 
rest  by  a  membranous  envelope  or  partition  wall;  (3)  a 
number  of  coalescent  cells,  consisting  of  protoplasmic 
units,  each  containing  one  nucleus,  but  the  units  not 
separated  from  each  other  by  cell  walls.  Such  a  cell  colony 


THE  PLANT  AND  ITS  FOOD      21 

of  sunlight,  which  enables  it  to  construct  com- 
plex carbon  compounds  when  supplied  with  the 
raw  materials,  carbon  dioxide  and  water.  In 
other  words,  the  chloroplast  is  a  mechanism 
which  is  able  to  build  up  carbon  compounds 
which  are  poorer  in  oxygen  than  are  the  raw 
materials  upon  which  it  works,  and  thus  the 
kinetic  energy  supplied  by  the  sunlight  be- 
comes converted  into  the  potential  energy  re- 
presented by  the  chemical  products  which  are 
formed  as  the  result  of  chloroplastid  activity. 
This  energy  (which  was  derived  from  the  sun), 
can  again  be  released  by  oxidation,  that  is 
by  more  or  less  rapidly  burning  those  products. 
It  may  then  be  utilised  to  heat  a  furnace,  to 
drive  a  steam  engine,  or  to  maintain  the  bodily 
processes  of  a  man. 

This  property  of  the  chloroplast  is  of  funda- 
mental importance,  not  only  for  the  plant, 
but  for  animals  as  well,  for  every  animal  is 
directly  or  indirectly  nourished  by  vegetable 

Products,  which  form  the  starting-point  of  the 
:>od-supply  of  the  world.  In  the  absence  of 
chlorophyll  there  would  be  none  of  the  higher 
plants  as  we  know  them,  nor  would  there,  in 
all  probability,  be  any  of  the  higher  animals 
at  all.  In  this  sense  we  are  indeed  all  children 


may  be  termed  a  syncytium.,  It  must  be  borne  in  mind 
that  the  membrane  is  not  an  essential  feature  of  cells, 
although  in  plants,  as  stated  in  the  text,  it  is  of  very  general 
occurrence.  The  cell  contains  other  Bodies,  e.  g.  chloro- 
plasts,  starch,  oils,  etc.,  but  these  are  non-essential,  and 
are  often  absent. 


22  PLANT  LIFE 

of  the  sun,  for  its  energy,  reaching  us  through 
the  mediation  of  the  plant,  is  the  jons  et 
origo  of  our  existence.  How,  then,  does  the 
Chlamydomonas  proceed  by  means  of  its 
chlorophyll  to  make  these  more  complex 
food  substances  ? 

Although  we  are  not  as  yet  fully  acquainted 
with  all  the  steps  of  the  process,  we  already 
know  enough  to  enable  us  to  sketch  in  out- 
line the  main  sequence  of  events.  Putting 
the  story  in  its  simplest  form,  we  may  say  that 
the  carbonic  acid,  which  is  formed  when 
carbon  dioxide  dissolves  in  water,  is  con- 
tinuously broken  up  as  the  result  of  the  action 
of  sunlight  of  a  suitable  intensity  upon  the 
chlorophyll  of  the  living  plant.  Oxygen 
is  liberated,  and  organic  compounds,  usually 
sugars,  are  produced  inside  the  cell.  When 
the  reaction  is  sufficiently  rapid,  so  that  the 
concentration  of  sugar  reaches  a  certain 
strength,  starch  often  makes  its  appearance, 
but  it  is  merely  a  secondary  product,  depending 
on  the  prior  formation  and  accumulation 
within  the  cell  of  sugar  in  sufficient  quantity. 
The  earlier  stages  of  sugar  formation  are  still 
obscure,  but  there  is  little  doubt  that  form- 
aldehyde (the  formalin  of  commerce)  is  pro- 
duced during  the  process.  This  substance  has 
been  used  as  the  starting-point  for  the  synthesis 
of  sugars  in  the  laboratory,  and  although  it 
is  difficult  to  detect  it  with  certainty  in  the 
plant  there  are  strong  reasons  for  considering 
that  it  really  is  formed  as  an  intermediate 


THE  PLANT  AND   ITS   FOOD      23 

product,  but  it  is  so  rapidly  changed  to  a 
more  complex  molecule  that  only  very  minute 
quantities  of  it  are  present  at  any  given  instant. 
Such  a  rapid  change  would  indeed  be  antici- 
pated, as  formaldehyde,  even  in  small  quan- 
tities, is  a  violent  poison,  that  is,  it  speedily 
reacts  with  ordinary  protoplasm  in  such  a  way 
as  to  destroy  the  intimate  chemical  archi- 
tecture of  the  living  substances.  The  mere 
fact  of  its  poisonous  character  constitutes 
no  objection  to  its  occurring  as  an  inter- 
mediate substance  in  the  synthetic  process; 
we  know  of  many  other  compounds  which, 
though  deadly  poisons  under  certain  circum- 
stances, are  still  normally  present  in  various 
phases  of  the  transmutation  of  substances 
going  on  within  the  plant  or  animal  body. 

Inasmuch  as  this  synthesis  of  sugar,  by 
means  of  the  chloroplast,  is  normally  dependent 
on  suitable  illumination,  the  process  is  com- 
monly called  Photosynthesis?  a  much  better 
term  than  the  older  expression,  Carbon  assi- 
milation, by  which  it  was  formerly  known. 

Since  Chlamydomonas  is  a  motile  organism, 
it  can  and  does  move  through  the  water  in 
which  it  lives  in  such  a  way  as  to  become 
exposed  to  the  best  conditions  of  illumination. 
This  faculty  of  taking  up  a  suitable  position 

1  Even  Photosynthesis  is  not  an  altogether  satisfactory 
term,  for  there  are  strong  reasons  for  believing  that 
although  light  starts  the  process,  it  is  not  concerned  in 
the  further  synthetic  processes  that  result  in  the  formation 
of  sugars. 


24  PLANT  LIFE 

as  regards  the  direction  of  the  light  is,  however, 
a  very  widespread  one,  and  we  shall  meet 
with  it  again,  although  in  a  modified  form,  in 
studying  the  behaviour  of  the  highest  plants ; 
for  the  property  of  irritability  which  in 
Chlamydomonas  finds  expression  in  the  inde- 
pendent movement  of  the  organism  as  a  whole, 
is  a  necessary  condition  of  existence  for  every 
plant  to  a  greater  or  less  extent.  Only  in 
this  way  is  it  possible  for  it  to  place  itself 
en  rapport  with  a  variable  and  changing 
environment,  and  hence  with  the  physical 
conditions  under  which  it  lives. 

A  brief  survey  of  the  more  salient  physical 
characters  of  chlorophyll  will  not  be  out  of 
place  here,  inasmuch  as  they  stand  in  sugges- 
tive relation  to  the  properties  of  this  remark- 
able substance. 

It  is  easy  to  extract  the  green  colour  of  plants 
by  soaking  a  quantity  of  grass  in  strong 
alcohol.  A  dark  green  liquid  will  thus  be 
obtained  which  if  examined  by  reflected  light 
will  appear  to  be  not  green,  but  blood-red  in 
colour.  This  property  of  "  fluorescence " 
is  not  confined  to  chlorophyll,  but  is  shared 
by  many  other  organic  and  some  inorganic 
substances,  and  it  affords  useful  hints  as  to 
their  more  intimate  chemical  architecture.  A 
solution,  prepared  by  the  rough-and-ready 
method  indicated  above,  is  of  course  not  pure 
chlorophyll,  but  it  contains  several  other 
colouring  matters  which  can  be  separated  from 
it  by  appropriate  means. 


THE  PLANT  AND  ITS  FOOD      25 

A  solution  of  chlorophyll  examined  by 
means  of  a  spectroscope  exhibits  a  number 
of  very  definite  absorption  bands,  due  to  the 
absorption  of  certain  of  the  coloured  rays 
of  the  spectrum  (=  the  dark  bands)  while 
the  rest  of  the  light  filters  through  and  is 
unaffected.  There  are  two  very  dark  bands 
in  the  red  region  of  the  spectrum,  and  others, 
mostly  fainter  and  more  diffuse,  in  the  yellow, 
green  and  blue-violet  regions.  Furthermore 
the  extreme  red  and  the  violet  end  of  the 
spectrum  are  also  obliterated. 

It  is  found,  as  might  perhaps  be  anticipated, 
that  the  rays  of  light  which  correspond  to  the 
dark  absorption  bands  in  the  red  region  of 
the  spectrum  are  those  principally  concerned 
in  promoting  photosynthesis.  The  other  rays 
which  are  absorbed  are  not  indeed  without 
influence,  but  they  are  of  comparatively  little 
consequence  from  the  point  of  view  with  which 
we  are  just  now  concerned.  We  see  from  the 
foregoing  why  it  is  so  essential  that  the  chloro- 
phyll in  the  living  plant  should  be  directly 
exposed  to  the  light  from  the  sky,  inasmuch 
as  any  light  which  has  already  traversed  a 
layer  of  chlorophyll  will  have  been  deprived 
of  those  rays  that  are  essential  for  photo- 
synthesis. Such  "  filtered"  light  will  of 
course  be  unable  to  develop  photosynthetic 
activity  in  a  chlorophyll-containing  organ  or 
organism  that  may  be  exposed  to  it.  The 
apparent  exceptions  afforded  by  plants  which 
flourish  in  deep  shade  are  due  to  the  circum- 


26  PLANT  LIFE 

stance  that  they  are  able  to  utilise  light  of  low 
intensity  which  pases  through  the  interspaces 
of  the  leaves  of  the  trees  above. 

It  turns  out  also  that  the  chlorophyll  green 
is  destroyed  by  the  same  rays  that  are  photo- 
synthetically  active,  and  this  destruction  is 
without  doubt  intimately  connected  with  its 
function  in  relation  to  such  rays.  It  does  not, 
however,  follow,  nor  indeed  is  it  probable,  that 
the  role  of  chlorophyll  is  a  very  direct  one  in 
influencing  all  the  stages  of  photosynthesis. 
It  is  more  likely  that  its  primary  function  is 
concerned  with  the  earliest  stages,  utilising 
the  energy  of  the  absorbed  light,  and  thus 
providing  the  conditions  for  starting  those 
processes  of  chemical  change  which,  under  the 
influence  of  protoplasm,  culminate  in  the 
formation  of  the  higher  carbon  compounds. 
For,  so  far  as  is  certainly  known,  it  is  only 
when  chlorophyll  is  united  with  the  living 
substance  that  these  higher  compounds  are 
able  to  make  their  appearance. 

When  the  green  chlorophyll  matter  is  decom- 
posed in  a  living  plant  cell,  other  colours, 
commonly  red  or  a  rusty  orange,  make  their 
appearance.  A  striking  example  of  this  is 
furnished  by  the  red  snow  plant,  Hcemato- 
coccus  nivalis.  This  unicellular  alga  is  closely 
related  to  species  of  Chlamydomonas,  and 
indeed  by  some  writers  is  included  in  that 
genus.  It  exists  in  a  green  and  a  red  form, 
and  is  either  motile,  like  Chlamydomonas, 
or  it  may  pass  into  a  non-motile  resting  stage, 


THE   PLANT  AND  ITS  FOOD      27 

when  it  will  withstand  complete  desiccation. 
When  it  is  grown  in  water  containing  traces  of 
available  nitrogenous  food  it  is  green,  thrives, 
and  multiplies  rapidly,  but  if  the  supply 
of  nitrogenous  food  is  used  up,  the  rate  of 
increase  drops,  and  the  plants  change  colour, 
owing  to  the  degradation  of  the  chlorophyll 
and  the  corresponding  development  of  a 
reddish  pigment.  When  it  has  reached  this 
condition  the  addition  to  the  water  of  a  small 
supply  of  nitrogenous  food,  such  as  a  crushed 
fly,  rapidly  brings  about  the  restoration  of 
the  green  colour  in  the  cells.  When  found 
growing  on  snow-slopes,  the  red  tint  is 
obviously  due  to  the  absence  of  available 
nitrogenous  food,  possibly  coupled  with  the 
conditions  of  intense  illumination  and  low 
temperature  prevailing  in  such  situations,  for 
when  the  plant  is  once  more  suitably  nourished 
the  green  colour  soon  re-appears. 

To  sum  up,  then,  what  we  have  learned  of* 
the  significance  of  chlorophyll,  both  to  the 
plant  and  to  the  world  at  large,  we  may  say 
that  its  primary  function  is  to  enable  its 
possessor  to  synthesise  important  complex 
foodstuffs  from  very  simple  raw  materials; 
in  other  words,  that  a  part  of  the  energy 
contained  in  the  sunlight  is  rendered  available 
for  the  use  of  the  plant.  Furthermore,  that 
the  sugars  or  their  representatives  thus 
formed,  provide  the  starting-point  for  still 
other  reactions  which  go  on  within  the  body. 
They  more  directly  supply  the  energy  which 


28  PLANT  LIFE 

renders  possible  the  production  of  those 
almost  infinitely  complex  substances  which 
form  the  very  substratum  of  life  itself. 


CHAPTER  III 

EVOLUTION  OF  CELLULAR  STRUCTURE  IN 
SIMPLE    PLANTS 

EMPHASIS  has  already  been  laid  on  the 
circumstance  that  the  plant  cell,  owing  to 
the  presence  of  its  investing  skin  of  cellulose, 
is  only  able  to  absorb  and  use  substances 
capable  of  diffusing  through  the  membrane. 
Consequently  food  from  without  can  only 
reach  the  protoplasm  in  solution.  Salts 
and  other  solid  food  materials  are  invariably 
absorbed  in  a  state  of  solution,  and  the  same 
is  true  of  gases,  such  as  oxygen  and  carbon 
dioxide,  as  well. 

But  water  has  other  functions  to  discharge 
within  the  plant,  besides  that  of  serving  as  a 
vehicle  for  the  intake  of  nutritive  materials. 
It  serves  to  maintain  the  protoplasm  itself 
in  that  characteristic  semi-fluid  condition 


CELLULAR  STRUCTURE  29 

which  is  essential  to  the  exercise  of  its  vital 
functions.  In  the  absence  of  sufficient  water- 
content  the  protoplasm  may  actually  die. 
Even  if  it  is  able  to  tolerate  comparative 
desiccation,  it  passes  into  a  state  in  which 
all  vital  reactions  are  slowed  down  until  they 
are  practically  brought  to  a  standstill. 

We  see  clearly,  therefore,  that  an  adequate 
water  supply  must  be  regarded  as  a  primary 
condition  of  all  active  life — prior  in  import- 
ance even  to  the  provision  for  photosynthesis 
in  the  green  plant.  For  when  water  is  cut 
off,  the  building  up  of  complex  carbon  com- 
pounds is  ipso  facto  arrested,  however  com- 
pletely all  other  conditions  of  photosynthesis 
may  be  satisfied. 

It  is  clear  that  plants  which  pass  the  whole 
of  their  lives  in  a  watery  medium  are  not 
confronted  with  the  risk  of  water  starvation, 
but  this  danger  may,  and  often  does,  become 
acute  as  soon  as  they  exchange  a  purely 
aquatic  for  a  terrestrial  habitat.  It  is  no 
exaggeration  to  assert  that  the  most  salient 
features  in  the  structure,  and  the  behaviour 
of  green  vegetation  in  general,  is  mainly 
connected  with  a  solution  of  the  problems  pre- 
sented by  water  requirements  on  the  one  hand, 
and  by  those  of  photosynthesis  on  the  other. 
It  is  in  relation  to  these  two  overwhelmingly 
important  functions  that  vegetation  has 
assumed  much  of  its  present  form,  for  any 
plant  failing  to  achieve  success  in  these  two 
directions  must  either  suffer  extinction,  or 


30  PLANT  LIFE 

must  obviate  the  inevitable  difficulties  of 
existence  in  another  way  altogether.  Some, 
like  fungi  and  many  parasites,  have  adopted 
the  latter  alternative;  but  as  regards  the 
vast  majority  of  the  green  plants,  we  shall 
find  that  a  recognition  of  these  two  dominating 
factors,  water  and  light,  will  serve  us  in  good 
stead,  as  furnishing  an  important  clue  to 
much  of  the  complexity  of  structure  to  be 
observed  in  so  many  of  the  more  advanced 
types  of  plants.  Such  complexity  is  intimately 
related  with  a  corresponding  differentiation 
and  specialisation  of  function,  and  indeed 
it  is  largely  to  this  circumstance  that  many  of 
the  more  striking  examples  of  "  adaptation 
to  the  environment  "  are  to  be  attributed. 

The  best  way  of  arriving  at  a  clear  concep- 
tion as  to  how  the  higher  plants,  with  their  com- 
plicated structure  and  high  degree  of  differ- 
entiation, have  come  into  existence  in  past 
times,  is  to  study  the  more  primitive  types 
which  illustrate  various  degrees  of  advance 
on  the  simple  unicellular  stage.  The  class 
of  Algae,  which  includes  the  simpler  water 
and  land  plants,  will  furnish  us  with  excellent 
material  wherewith  to  reconstruct,  in  outline 
at  least,  the  course  of  vegetative  evolution. 
This  must  not,  however,  be  taken  to  imply 
that  the  simple  types  in  question  are  to  be 
regarded  as  new  forms  which  are  now  on  an 
upward  grade  of  evolutionary  development. 
They  are  to  be  understood,  rather,  as  per- 
manent representations  of  some  of  the  phases 


CELLULAR  STRUCTURE  31 

through  which  the  ancestors  of  the  higher 
plants  have  almost  certainly  passed.  They 
may  be  regarded  as  morphological  survivals 
that  have  slipped  out  from  the  main  stream 
of  evolution  into  the  quiet  backwaters  of  life, 
preserving  in  themselves  the  types  and  forms 
of  a  vegetation  that  otherwise  might  well 
have  passed  into  oblivion.  Furthermore, 
whilst  they  may  be,  and  often  are,  admirably 
fitted  by  their  very  simplicity  and  variety 
to  certain  kinds  of  surroundings,  they  are 
not  suited  for  a  life  under  other  conditions 
which  demand  a  more  highly  specialised 
body. 

As  a  matter  of  fact  it  is  far  from  easy 
to  define  very  exactly  what  is  meant  by 
44  higher  "  and  "  lower  "  types  respectively. 
We  commonly  associate  the  ideas  of  spe- 
cialisation and  differentiation  with  the  higher 
types.  An  obvious  adaptation  to  a  particular 
environment  is  often  taken  as  a  sign  of  high 
organisation,  but  in  reality  very  many  of  the 
extremely  simple  plants  are  admirably  adapted 
to  their  particular  surroundings.  Moreover, 
we  are  acquainted  with  numerous  species 
which  at  the  present  time  are  simple  because 
they  have  lost  the  complexity  of  structure 
formerly  possessed  by  their  ancestors.  We 
often  speak  of  these  as  degraded  forms ;  but 
parasites,  which  illustrate  this  point  very 
well,  are  frequently  admirably  adapted  by 
their  very  simplicity  of  structure  to  their 
particular  modes  of  life.  In  practice, 


32  PLANT  LIFE 

however,  we  can  generally  distinguish  between 
what  is  primitively  and  what  is  secondarily 
simple,  and  all  that  need  be  said  here  about 
the  matter  is  that  in  any  classification  of  this 
sort  most  people  more  or  less  unconsciously 
adopt  an  anthropomorphic  standpoint  and 
standard.  Provided  we  recognise  this  for 
ourselves,  we  shall  avoid  confusion  of  thought, 
and  our  mental  picture  will  be  the  clearer. 

Within  the  genus  Chlamydomonas,  which 
we  selected  as  an  example  of  a  primitive 
plant,  we  find  that  some,  at  any  rate,  of  the 
species  are  able  to  manifest  a  change  of  form 
and  character  according  to  the  circumstances 
under  which  they  are  growing.  This  fact 
will  serve  as  a  starting-point  from  which 
to  trace  the  development  of  complication  of 
form  and  structure  in  the  plant  kingdom. 

Individuals  belonging  to  certain  species, 
e.  g.  Chlamydomonas  Braunii,  when  cultivated 
on  appropriate  nutritive  media,  such  as  a 
relatively  concentrated  solution  of  mineral 
salts,  or  on  a  damp  substratum,  cease  to 
multiply  in  the  ordinary  way.  Instead  of 
the  cells  which  have  been  formed  by  the 
division  of  a  parent-cell  becoming  separated 
and  swimming  away,  they  remain  cohering 
together.  Their  cellulose  walls  swell  up  and 
form  a  gelatinous  mass  in  which,  as  in  a 
matrix,  the  cells  (i.  e.  the  nucleated  proto- 
plasmic units)  which  have  arisen  by  the 
repeated  fission  of  parent  cells  remain  em- 
bedded. Even  the  cilia  may  become  en- 


CELLULAR  STRUCTURE  33 

veloped  by  the  swelling  jelly,  and  they  may 
even  entirely  disappear.  The  cell  colonies 
thus  consist  of  motionless  masses  of  green 
jelly.  But  there  is  as  yet  very  little  organisa- 
tion in  such  colonies.  The  form  of  the  mass 
is  not  constant,  and  a  return  to  what  may  be 
termed  normal  conditions  of  life  may  readily 
lead  to  a  complete  disruption  of  the  colony, 
the  individual  cells  escaping  from  the  jelly 
and  returning  to  the  unicellular  motile  condi- 
tion which  is  in  the  main  characteristic  of  this 
group  of  algae. 

This  tendency  to  form  agglomerations  of  in- 
dividual cells  is  carried  to  a  far  greater  degree 
of  perfection  in  some  other  groups  of  the  lower 
algae.  Thus  Apiocystis  Brauniana  (Fig.  2), 
an  alga  fairly  often  to  be  met  with  in  ponds, 
attached  to  larger  algae  and  other  objects, 
consists  of  a  pear-shaped  mass  of  jelly  in 
which  are  scattered  masses  of  chlorophyll- 
containing  protoplasm.  The  little  proto- 
plasmic spheres,  which  represent  the  living 
part  of  the  individual  cells  composing  the 
Apiocystis  plant,  are  grouped  in  the  more 
peripheral  parts  of  the  gelatinous  matrix 
formed  by  the  swelling  of  the  common  cell 
walls.  Each  cell  (see  footnote  on  p.  20)  is 
furnished,  like  Chlamydomonas,  with  a  pair 
of  cilia,  but  these  have  ceased  to  be  functional, 
for  they  are  enclosed  in  a  thin  projection 
of  the  gelatinous  wall.  When  examined 
attentively,  it  can  be  observed  that  the 
individual  cells,  or  protoplasts,  are  multiplying 


34 


PLANT  LIFE 


by  fission.  This  is  betrayed  by  their  aggrega- 
tion in  pairs  and  groups  of  cells  which  have 
obviously  sprung  from  a  single  parent.  But 
an  interesting  and  important  feature  of  their 
development  consists  in  the  fact  that,  after 


II 


Fig.  2.  —  Apiocystis  Brauniana.  I,  —  Mature  Plant. 
II. — Younger  Plant.  G,  The  gelatinous  outer  layer  of  the 
membrane. 

they  have  been  formed,  they  can  modify 
their  positions.  Thus  all  of  them  come  to  lie 
just  beneath  the  periphery  of  the  jelly,  and 
are  not  uniformly  distributed  through  it  as 
might  have  been  expected.  This  arrange- 
ment is  one  which  secures  evident  advantages 
from  the  point  of  view  of  suitable  illumination. 


CELLULAR  STRUCTURE  35 

We  see  here  a  simple  example  of  co-ordination 
between  the  cells  of  an  organism.  It  is  true 
that  it  is  of  a  very  rudimentary  kind,  but 
the  fact  that  an  organism,  originating  in  this 
manner,  possesses  a  definite  form  at  all,  is 
a  clear  proof  of  its  existence. 

The  form  of  Apiocystis  seems  to  be  fairly 
constant,  but  when  conditions  are  suitable, 
some  or  all  of  the  protoplasts  may  escape 
from  the  gelatinous  sheath  and  swim  away 
as  biciliate  Chlamydomonas-like  organisms, 
though  they  are  destitute,  for  a  time  at  least, 
of  even  a  cell  membrane.  In  this  condition 
they  are  known  as  Zoospores ;  when  one  of 
them  settles  down  it  becomes  invested  in  a 
cell  wall  secreted  by  the  protoplasm,  and 
by  repeated  fission  builds  up  another  Apio- 
cystis plant.  This  mode  of  reproduction  by 
means  of  zoospores  is  very  common  in  the 
algae,  and  it  serves  to  recall  the  early  stages 
in  the  history  of  the  race  which  is  thus  re- 
peated during  the  beginning  of  the  life  of  a 
new  individual. 

Now  a  pear-shaped  organism  is,  by  its 
very  form,  rendered  incapable  of  reaching 
a  large  size,  at  any  rate  without  such  accessory 
complications  as  are  not  to  be  thought  of  in 
connection  with  primitive  plants.  There  are 
other  lines  of  development  which  have  proved 
more  fruitful  from  an  evolutionary  point 
of  view,  and  of  these  the  flattened  expansion 
and  the  filamentous  types  represent  the  most 
successful.  Indeed,  it  is  on  these  lines,  or 


36  PLANT  LIFE 

rather  on  a  combination  of  both  of  them, 


Fig.  3. — Ulva  lactuca,  the  Sea -Lettuce. 

that  the  development  of  the  higher  forms  of 
vegetation  has  mainly  advanced. 


CELLULAR  STRUCTURE  37 

The  group  of  algae  to  which  belongs  the 
green  sea-lettuce,  so  common  on  some  of  our 
coasts,  especially  where  the  sea-water  is  con- 
taminated by  sewage  effluents,  furnishes 
beautiful  examples  of  the  simpler  stages  in 
the  evolution  of  a  flattened  leaf-like  type  of 
thallus,  Ulva,  the  sea-lettuce  in  question 
(Fig.  3),  is  somewhat  advanced,  for  it  consists 
of  cells  which  are  arranged  in  two  layers,  but 
otherwise  division  occurs  in  the  cells  of  each 
layer  in  such  a  way  as  to  increase  the  area  of 
the  surface.  The  multiplication  of  cells  is  not 
very  uniform  over  the  whole  surface,  those 
nearer  the  margins  dividing  and  growing 
faster  than  those  nearer  the  middle  line  of 
the  leaf-like  plant.  Thus  the  surface  of  the 
frond  is  thrown  into  folds  and  wrinkles  as 
the  necessary  consequence  of  this  unequal 
growth,  But  that  there  is  some  co-ordinating 
influence  at  work  amongst  the  cells  is  shown 
by  thejact  that  this  wrinkling  does  not  become 
excessive,  and  the  plants  assume  a  fairly  defi- 
nite form  which  makes  any  given  individual 
easy  to  recognise  as  belonging  to  this  and  no 
other  species.  The  Ulva  plants  are  securely 
anchored  to  stones  and  other  supports  by 
a  special  development  of  the  cells  near  the 
base  of  the  plant.  These  grow  out  into  long 
filamentous  strands,  and  adhere  very  closely 
to  the  surface  of  the  rock.  The  specimens 
one  often  sees  washed  up  after  a  storm  «are 
usually  the  upper  parts  of  the  plants,  which 
have  become  torn  off  by  the  waves. 


38  PLANT  LIFE 

So  far  we  have  only  considered  aquatic 
forms  of  algae,  but  there  are  certain  kinds 
which  grow  in  damp  situations  on  land, 
Amongst  these  is  Prasiola,  which  is  not  un- 
frequent  in  certain  localities  (Fig.  4).  Its  body 
is  composed  of  a  leaf-like  expansion  of  cells 
which  lower  down  form  a  contracted  stalk-like 


Fig.  4. — Prasiola  stipitata.  I. — General  appearance  of  plant 
magnified  about  four  times.  II. — Portion  of  frond  magni- 
fied 300.  The  living  cell  contents  are  embedded  in  the 
flattened  jelly  which  originates  by  the  swelling  and  growth 
of  the  cell  walls. 

body  attached  to  the  soil  by  means  of  special 
filamentously  elongated  cells  called  rhizoids. 
The  cells  which  compose  the  substance  of  the 
thin  leaf-like  body  are  all  alike,  but  the 
common  walls,  as  befits  a  terrestrial  organism, 
are  more  cartilaginous  and  tough  than  those 
of  the  more  aquatic  types.  Even  repro- 
jduction  is  correlated  with  the  change  of 
1  habitat  from  water  to  the  land.  The  cells 
which  become  detached  from  the  frond  are 


CELLULAR  STRUCTURE  39 

not  motile,  and  the  plant  is  furthermore 
increased  by  budding  and  by  outgrowths  from 
detached  portions  of  thallus.  The  cells  are 
very  regularly  arrayed  in  the  thallus,  and  the 
conformation  of  the  plant  is  clearly  the  result 
of  a  co-ordination  existing  between  the  con- 
stituent cells,  and  this  is  of  a  tolerably 
advanced  nature.  Indiscriminate  multiplica- 
tions of  the  individual  cells  has  been  replaced 
by  a  more  ordered  and  ^regulated  distribution 
of  the  power  of  division  amongst  the  cells 
which  together  make  up  the  Prasiola  plant. 
The  relation  of  the  frond  to  light  is  one  of 
the  important  factors  in  bringing  this  about. 
But  there  is,  besides  this,  and  perhaps  behind 
it,  a  subtle  intercommunicating  influence 
between  the  individual  cells  which  together 
constitute  the  colony,  and  this  influence 
determines  the  share  that  each  is  to  take  in 
the  building  up  of  the  organism  as  a  whole. 
Although  we  may  find  it  impossible  to  identify 
the  exact  nature  of  this  influence  in  the 
majority  of  instances,  we  know  quite  enough 
to  convince  us  that  it  is  of  a  material  nature. 
We  are  well  aware  that  the  processes  of  growth, 
and  many  other  bodily  functions  besides,  are 
greatly  affected  by  the  presence  of  even 
minute  traces  of  certain  substances,  and  the 
physical  approximation  of  the  cells  of  an 
organism  renders  it  possible  for  substances 
to  diffuse  from  one  to  the  other,  and  thus 
to  determine,  in  a  plus  or  minus  direction, 
the  rate  of  cell  growth  and  multiplication. 


40  PLANT  LIFE 


CHAPTER  IV 

THE  CELLS  AND  THE  ORGANISM 

TURNING  from  the  flattened  forms  to  the 
filamentous  types  of  algae,  we  find  a  great 
variety  of  forms,  accompanied  by  a  very 
different  degree  of  autonomy  in  the  constituent 
cells  of  the  filamentous  body.  Moreover,  we 
see  very  clearly  that  closely  analogous  forms 
have  been  reached  by  several  evolutionary 
routes.  In  other  words,  much  the  same 
kind  of  organisation  may  have  been  arrived 
at  by  plants  which  have  descended  from 
several  diverse  simple  stocks.  This  con- 
vergence of  type,  or  analogous  similarity 
between  remotely  related  forms,  is  of  fairly 
wide  occurrence  both  in  animals  and  in  plants, 
nor  is  it  by  any  means  restricted  to  the 
simpler  members  of  either  kingdom. 

Our  first  illustration  of  an  alga  organised 
on  the  filamentous  plan  is  afforded  by  the 
species  known  as  Hy drums  jcetidus  (Fig.  5).  It 
is  an  aquatic  plant,  rare  in  Britain,  but  fairly 
abundant  in  many  Alpine  rills  near  the 
melting  s-now.  The  reason  of  this  is  that  the 
alga  only  thrives  at  a  low  temperature,  soon 
perishing  in  water  above  12°  C.  (=  about 
54°  F.).  The  plants  are  rather  plumosely 


CELLS  AND  THE  ORGANISM       41 

branched    above,    and    the    branchlets    are 


g.  5. — Hydrurus  fcetidus.  I. — General  character  of  the 
plant.  '  II. — One  of  the  tips  of  the  branches  highly  magni- 
fied showing  the  cells  of  which  the  plant  is  built  up.  The 
protoplasmic  bodies  are  embedded  in  a  common  jelly 
formed  by  the  swelling  up  of  the  walls  or  membranes,  but 
the  innermost  layer  forms  a  sharply  defined  skin  round 
each  protoplast. 

hairy  "  or  villous.    The  lower  part  forms 


42  PLANT  LIFE 

a  smooth  stalk,  and  is  attached  to  stones,  etc., 
by  a  slightly  expanded  disc-like  foot. 

An  individual  Hydrurus  is  made  up  of  a 
colony  of  unicellular  algae,  the  walls  of  which 
have  become  swollen  and  rather  firmly 
gelatinous.  The  whole  organisation  of  the 
plant  depends  on  the  different  mode  of 
development  followed  by  the  various  in- 
dividual cells.  The  apices  of  all  the  branches 
and  hair-like  protuberances  are  occupied  by 
single  cells,  and  it  is  to  these  that  the  alga 
owes  its  definite  form.  The  terminal  cells 
of  the  branches  multiply  by  dividing  longi- 
tudinally; one  of  the  two  daughter  cells  then 
gradually  slides  in  front  of  the  other  and 
continues  to  function  as  the  growing  apex, 
the  other  one,  which  has  taken  a  rearward 
position,  contributes  to  the  building  up  of 
the  plant  body.  Some  of  these  cells  behind 
the  apex  extend  outwards  from  the  cylindrical 
surface  and  become  the  starting-points  of 
new  branches;  or,  if  their  growth  is  but 
limited,  they  merely  give  rise  to  the  villous 
hairy  outgrowths. 

The  important  lesson  to  be  learned  from 
Hydrurus  is  that  a  definite  co-ordination 
exists  amongst  the  individuals  composing 
the  colony  or  association.  In  this  way  it  be- 
comes possible  to  speak  of  the  terminal  cells 
as  "  apical  cells  " ;  that  is  to  say,  they  have 
assumed  the  role  of  determining  the  fashion 
of  the  branching,  the  rest  of  the  cells  merely 
building  up  the  plant  on  the  lines  laid  down 


CELLS   AND   THE   ORGANISM       43 

at  these  apices.  Although  Hydrurus  recalls 
other  algae  already  described,  in  so  far  as  it 
consists  of  an  organised  cell  colony,  it  is  very 
far  removed  from  a  near  relationship  with 
them,  for  it  belongs  to  quite  a  distinct  group. 
The  cells  are  of  a  yellow  colour,  and  when 
the  protoplasts  escape  from  their  containing 
gelatinous  walls  they  only  possess  one  cilium 
instead  of  two  as  in  Chlamydomonas. 

The  majority  of  the  filamentous  algae  are 
composed  of  cells  of  an  elongated  form,  placed 
end  to  end,  and  the  colonial  origin  of  such 
plants  is  more  and  more  obscured  owing  to  the 
specialisation  which  takes  place  amongst  the 
cells,  for  these  gradually  cease  to  form  merely 
coherent  but  obviously  distinct  units.  They 
come  to  exist  as  mere  parts  of  a  higher 
organisation,  the  latter  more  and  more  control- 
ling the  arrangement  and  development  of  the 
constituent  cells.  Thus  the  relative  import- 
ance of  the  cell  and  the  organism  is  gradually 
reversed.  In  the  lower  types  it  is  not  always 
easy  to  discover  the  organism  in  a  congeries 
of  cells,  whilst  in  the  higher  ones  the  control- 
ling organisation  of  the  complex  individual 
may  almost  completely  override  the  independ- 
ence of  the  constituent  cells. 

As  an  example  of  a  plant  the  cells  of  which 
have  still  retained  a  considerable  measure  of 
autonomy,  we  may  name  Spirogyra,  one  of 
the  commonest  of  the  thread-like  algae  to  be 
met  with  in  ponds  and  ditches,  where  it  is 
easily  recognised  by  its  bright  green  colour  and 


44  PLANT  LIFE 

the  slippery  gelatinous  character  of  its  mem- 
branes. The  cells  of  the  filaments  are  com- 
monly elongated,  but  each  one  behaves  very 
much  as  an  independent  unit.  The  effect  of 
one  cell  upon  another  is  of  the  slightest  under 
ordinary  circumstances.  Each  divides  trans- 
versely, and  so  multiplies,  independently  of 
its  neighbours.  The  filament  thus  grows  in 
length,  but  it  usually  has  no  distinguishable 
base  or  apex,  nor  does  it  branch.  Altogether 
the  organising  effect  of  the  cell  union  is  as 
yet  of  the  very  simplest  kind. 

Another  common  alga,  Cladophora,  presents 
quite  a  different  state  of  affairs  (Fig.  6).  This 
plant,  like  the  foregoing,  consists  of  cells  placed 
end  to  end,  but  there  the  similarity  ceases. 
Each  cell  is  definitely  part  of  the  organism. 
The  filament  is  attached  by  a  specialised  basal 
cell  and  it  increases  in  length  solely  by  trans- 
verse division  of  the  apical  cell.  Branches 
may  spring  from  the  cells  behind  the  apex, 
and  they  then  commonly  appear  in  regular 
sequence,  the  youngest  branches  arising  as 
outgrowths  from  the  anterior  (e.  g.  nearer  the 
growing  point  of  the  stem)  end  of  the  cell 
nearest  the  apex. 

Not  only,  therefore,  is  the  plant  as  a  whole 
organised  in  such  a  way  that  there  is  a  base, 
as  distinct  from  an  apex,  but  this  distinction 
is  also  impressed  on  every  cell l  which  helps 

1  In  a  certain  sense  the  expression  "  cell "'  is  not 
appropriate  to  the  structural  unit  of  Cladophora,  since 
each  "  cell  "  really  represents  a  syncytium  (p.  21)  because 
its  protoplasm  contains  several  nuclei. 


CELLS  AND  THE  ORGANISM       45 

to  make  up  the  Cladophora  plant.  It  may 
easily  be  shown  that  the  co-ordination  is 
a  real  one,  and  that  it  depends  in  some  way 
on  the  mutual  reactions  between  one  cell  and 
another,  by  means  of  an  experiment  on  one 
of  the  marine  species  of  the  genus.  If  the 


Fig.  6. — Cladophora  sp.     I. — General  habit.     II. — Magnified 
filament. 

plant  be  placed  in  sea-water  to  which  a 
strong  salt  solution  is  slowly  added  (up  to 
about  12*5  %),  the  protoplasm  of  each  of 
the  cells  contracts  away  from  the  walls,  and 
forms  an  ellipsoidal  mass  within  each  cell 
cavity.  The  protoplasts  then  surround  them- 
selves with  new  walls.  After  allowing  them  to 


46  PLANT  LIFE 

remain  in  this  condition  for  about  four  days 
the  strong  salt  solution  is  gradually  replaced 
by  sea-water.  The  ellipsoidal  cells  swell  out 
until  they  approximately  fill  the  space  they 
previously  occupied  within  the  old  membrane. 
But  the  disunion  has  obliterated  the  mutual 
relationship  formerly  existing  between  cell 
and  cell.  Each  one  proceeds  to  develop  with- 
out any  reference  to  the  rest,  and  puts  out  a 
basal  attaching  organ  below,  which  usually 
penetrates  the  adjacent  cell  cavity.  Later 
on  the  cell  also  proceeds  to  grow  in  the 
apical  direction.  Thus  the  individual  cells 
of  the  filamentous  colony  have  been,  in  this 
experiment,  released  from  the  influence  which 
bound  them  together  into  an  organism,  and 
have  recovered  complete  autonomy  and  in- 
dividuality. This  has  occurred  as  the  result 
of  sundering  the  protoplasts  from  all  com- 
munication with  one  another  for  the  period 
of  time  during  which  they  remained  con- 
tracted in  the  strong  salt  solution. 

An  experiment  such  as  this  is  specially 
valuable,  since  it  enables  us  to  appreciate 
not  only  the  reality  of  the  continuous  inter- 
change of  material  between  cells  that  are 
in  close  contact,  whereby  co-ordination  is 
rendered  possible,  but  it  serves  to  show  how 
closely  this  co-ordination  to  form  an  organism 
is  bound  up  with  such  mutual  interchange. 
For  the  protoplasts,  although  separated  by 
membranous  cell  walls  from  each  other, 
are  yet  in  intimate  connection;  in  many 


CELLS   AND   THE  ORGANISM       47 

instances  connecting  strands  of  protoplasm 
have  been  demonstrated,  and  these  serve  as 
the  obvious  channels  of  direct  communication 
between  the  living  contents  of  adjacent  cells. 
When  the  interchange  has  been  sufficiently 
interrupted  the  old  order  cannot  be  again 
restored.  The  cells  are  released,  as  it  were, 
from  the  influence  that  previously  controlled 
them  and  caused  them  to  be  welded  together 
into  a  higher  individuality.  Each  cell,  thus 
breaking  away  from  the  union,  reverts  to 
a  more  primitive  condition,  recovering  an 
independence  akin  to,  and  perhaps  identical 
with,  that  which  distinguishes  zoospores 
and  other  reproductive  cells  that  are  set 
free  from  the  organism  which  gives  them 
birth. 

Although  the  simpler  filamentous  algae, 
and  especially  the  branching  kinds,  share  with 
the  primitive  flattened  leaf-like  types  the 
advantage  of  disposing  their  surfaces  so  as 
to  make  the  most  of  the  means  of  illumin- 
ation, they  yet  remain  far  behind  the  more 
advanced  types,  in  which  other  functions 
beside  those  of  photosynthesis  press  for 
notice. 

The  larger  seaweeds,  although  their  green 
colour  is  masked  by  yellow  or  red  pigment, 
are  as  dependent  on  light  for  the  manu- 
facture of  their  food  as  are  their  simpler 
green  companions.  But  their  size  introduces 
an  element  of  physiological  complexity. 

It  will  be  remembered  that  it  is  only  the 


48  PLANT  LIFE 

directly  illuminated  cells  which  take  an 
active  share  in  photosynthesis.  What  is  the 
use,  then,  of  those  vast  numbers  of  internal 
cells  which  lie  beneath  the  outer  surface  of 
a  large  seaweed,  and  constitute  its  main 
bulk  ?  Let  us  examine  one  of  the  big  sea- 
weeds, for  example  Laminaria,  which  form 
the  large  leathery  strap -like  plants  growing 
below  the  tide  limits.  We  shall  find  it 
consists  of  a  stout  stalk,  firmly  adhering  by 
a  specialised  base  to  the  rocks,  and  thinning 
out  abruptly  above  to  form  the  flattened 
frond.  The  cells  which  compose  the  plant 
are  by  no  means  all  alike,  and  at  least  three 
different  kinds  can  be  distinguished.  First 
there  are  the  crowded,  rather  small  ones, 
forming  the  superficial  layers.  These  are 
those  chiefly  concerned  in  photosynthesis. 
Beneath  the  outer  layers  are  other  cells, 
larger  and  more  irregular  in  shape.  These 
are,  partly  at  least,  concerned  in  storing  up 
the  surplus  products  of  photosynthesis. 
Thirdly,  in  the  more  central  regions  of  the 
massive  stalk  are  to  be  found  strands  of 
very  much  elongated  cells  which  clearly  serve 
as  conducting  elements.  In  some  of  the  larger 
seaweeds  the  cross  partition  walls  between 
these  cells  are  visibly  perforated,  thus  admit- 
ting of  still  easier  passage  of  soluble  contents 
along  their  course.  Some  of  these  large 
brown  seaweeds  recall  the  habit  of  our 
terrestrial  plants  in  that  they  even  throw 
off  their  "  leafy  "  portions  periodically,  and 


THE   'NON-CELLULAR'   TYPE     49 

produce  new  ones  by  the  rapid  division  of 
the  cells  in  the  region  between  the  stalk  and 
the  base  of  the  expanded  frond. 


CHAPTER  V 

THE     "  NON-CELLULAR  "    TYPE    OF 
ORGANISATION 

IN  the  series  of  plants  hitherto  considered 
we  have  been  mainly  concerned  in  tracing 
certain  lines  of  evolution  in  form  and  structure, 
accompanied  by  a  corresponding  differentia- 
tion and  specialisation,  amongst  the  cells  of 
which  the  bodies  of  the  plants  are  constructed. 
These  culminate  in  such  forms  as  the  higher 
red  and  brown  seaweeds  in  which  the  char- 
acter of  leafy  plants  is  very  closely  imitated, 
save  in  one  important  respect.  The  terrestrial 
plant,  unlike  the  submerged  seaweed,  is 
exposed  to  difficulties  connected  with  the 
water  supply,  and,  as  we  shall  subsequently 
see,  this  has  necessitated  structural  and  other 
developments  far  in  advance  of  those  ex- 
hibited by  aquatic  plants.  Indeed,  a  high 


50 


PLANT  LIFE 


degree  of  cellular  differentiation  is  really  not 
essential  for  water  plants,  and  we  shall  find 
that  the  complex  structure  of  terrestrial 
species  becomes  simplified  in  any  descendants 
that  may  have  taken  to  a  watery  habitat. 

Even  amongst  the  algae  high  differentiation 
of  external  form  is  not  necessarily  associated 


IJTT 


Fig.  7.—Caulerpa  StaUii. 

with  a  cellular  complexity  of  corresponding 
magnitude.  This  is  well  seen  in  those  sea- 
weeds that  consist  of  a  large  number  of  cells 
which,  though  enclosed  in  a  common  peri- 
pheral membrane  of  cellulose,  are  not  parti- 
tioned off  from  each  other  by  cell  walls.  An 
example  of  such  a  plant,  which  combines 
a  somewhat  highly  differentiated  external 
form  with  an  internal  structure  of  remarkable 


THE  'NON-CELLULAR'  TYPE      51 

simplicity,  is  furnished  by  the  seaweed  known 
as  Caulerpa  Stahlii.  As  is  shown  by  the 
annexed  illustration,  the  plant  consists  of 
a  creeping  stem  from  which  arise  the  erect 
leafy  expansions ;  while  the  whole  is  anchored 
by  root-like  structures  which  penetrate,  or 
adhere  to,  the  substratum.  In  spite  of  this 
high  degree  of  external  differentiation,  there 
is  no  internal  partitioning,  and  no  one  of  the 
vast  number  of  nucleated  protoplasts,  which 
together  make  up  the  living  substance,  is 
segregated  physically  from  its  neighbours  by 
obvious  boundaries.  But  there  is  one  signifi- 
cant and  interesting  feature  about  the  dis- 
tribution of  the  nuclei  in  the  protoplasm. 
They  are  crowded  at  the  growing  points, 
and  are  more  widely  spaced  asunder  in  the 
older  regions.  In  this  apparently  trivial 
circumstance  we  can  discern  exactly  the 
same  arrangement  as  would  have  been  ob- 
served had  the  partitioning  walls  been  actually 
present,  for  the  dividing  cells  in  the  growing 
points  always  appear  to  be  both  numerous 
and  small,  owing,  of  course,  to  the  rapid 
cell  division  which  is  going  on  in  such  regions. 
Now  this  "  non-cellular  "  or  "  syncytial " 
(see  p.  21)  type  of  organisation  entails 
certain  obvious  disadvantages  on  its  possessor, 
but  we  find  that  in  some  instances  the  at- 
tendant risks  have  been  overcome  in  a 
wonderful  way.  On  coral  reefs  and  similar 
calcareous  stations  an  alga  known  as  Halimeda 
Opuntia  is  sometimes  found  (Fig.  8).  It  resem- 


52  PLANT  LIFE 

bles  in  its  general  shape  a  small  Prickly  Pear 


Fig.  8. — Halimeda  Opuntia.  I. — General  appearance  of  the 
plant.  II. — A  section  through  one  of  the  flattened  lobes  to 
show  the  palisade-like  peripheral  branches  passing  into 
the  broader  longitudinal  filaments  or  strands. 

or  Opuntia.     From  an  attaching  organ  there 
rises  a   jointed    and    much -branched   plant. 


THE   'NON-CELLULAR'   TYPE      53 

The  whole  is  strongly  impregnated  with  a 
deposit  of  lime,  whereby  the  plant  acquires 
a  considerable  degree  of  strength  and 
rigidity.  The  remarkable  cactus-  or  opuntia- 
like  form  is  produced  by  a  wonderful  weaving 
together  of  the  branching  filaments  of  which 
the  whole  plant  is  made  up.  There  are  no 
traces  of  cross  walls  in  these  tubular  branches, 
but  there  is  a  considerable  difference  between 
the  different  regions  of  the  branches  them- 
selves. The  lower  part  of  each  branch 
system  runs  down  the  centre  of  the  plant, 
while  the  final  short  twigs  form  the  outer 
surface  of  the  flattened  segments.  These 
final  branchlets  are  closely  adherent,  and  the 
flattened  segment  of  the  plant,  looked  at 
from  the  outside,  seems  to  be  clothed  with 
a  mosaic  of  small  cells — these  being,  of  course, 
the  tips  of  the  branches  just  mentioned. 
Not  only  this,  but  the  chlorophyll  is  almost 
entirely  confined  to  these  peripheral  branch- 
lets,  whilst  the  hinder  and  wider  parts  of  the 
tubes  serve  to  store  and  distribute  the  food 
material  manufactured  in  the  tips  when 
exposed  to  light. 

Halimeda,  then,  furnishes  a  wonderful 
example  of  co-ordinated  growth.  The  singular 
completeness  with  which  it  has  solved  the 
problem  of  attaining  a  very  high  degree  of 
specialisation  with  the  simplest  materials, 
extends  to  every  detail  of  its  structure.  It 
is  admirably  adapted,  both  to  utilise  the  light 
and  to  store  away  the  material  products  of 


54  PLANT  LIFE 

photosynthesis;  whilst  it  has  overcome  the 
disabilities  apparently  inherent  in  its  type 
of  organisation,  by  strengthening  and  cement- 
ing together  the  branching  filaments,  of  which 
it  is  built  up,  by  means  of  the  calcium  car- 
bonate which  it  withdraws  from  the  sea-water. 

The  consideration  of  the  noncellular  or 
syncytial  plant  has  been  introduced  in  order 
to  illustrate  the  varieties  of  one  possible 
type  of  structure.  Save,  however,  for  the 
production  of  a  few  aquatic  representatives 
it  does  not  mark  a  line  of  important  advance. 
The  multicellular  condition  contained  within 
itself  the  promise  of  the  future,  and  it  is  as 
multicellular  organisms  that  the  higher  plants 
have  been  evolved. 

It  will  be  useful  at  this  point  to  sum  up  the 
salient  points  of  the  preceding  discussion, 
so  as  to  gain  a  clear  starting-point  from  which 
to  study  the  evolution  and  modification  of 
form  and  structure  in  the  higher  terrestrial 
forms  of  life. 

We  have  seen  the  striking  consequences 
which  accrue  from  the  possession  of  an  invest- 
ing membrane  in  their  effects  upon  the  mode  of 
nutrition,  and  indirectly  upon  other  functions, 
e.  g.  that  of  motility,  in  plants.  We  have  learnt 
in  the  relatively  lowly  members  of  the  vege- 
table kingdom  which  have  been  passed  under 
review,  why  a  need  for  the  presentation  of 
the  green  surfaces  to  light  should  be  a  matter 
of  such  cardinal  importance  as  to  dominate 
the  organisation  of  every  one  of  them.  We 


THE   'NON-CELLULAR'  TYPE      55 

have  also  followed  out  the  gradual  loss  of 
motility,  and  the  coherence  of  the  individual 
cells,  for  a  period  of  their  lives  at  any  rate. 
We  have  furthermore  recognised  the  fact 
that  there  exists  a  mutual  influence,  of  a 
material  kind,  which  leads  to  the  co-ordination 
of  the  cells  of  a  colony  in  such  a  way  as  to 
produce,  not  a  mere  congeries  of  separate 
entities,  but  an  organism.  In  other  words,  we 
have  traced  the  gradual  curtailment  of  the 
individuality  characteristic  of  the  primitive 
cells,  and  have  witnessed  the  corresponding 
transference  of  it  to  the  cell  colony  as  a  whole. 
This  transference  of  individuality  is  intimately 
connected  with  physiological  correlation, 
which  is  doubtless  exerted  through  functional 
and  material  agencies — largely  by  modifica- 
tions in  the  nutritive  processes — with  the 
result  that  each  cell  unit  is  intimately  affected 
by  what  is  going  on  in  its  neighbours,  as  well 
as  in  other  and  more  remote  regions  of  the 
organism;  the  final  result  is  that  the  cell 
tends  to  become  more  definite  and  circum- 
scribed in  form,  and  more  limited  and  special- 
ised in  function.  To  put  it  a  little  differently, 
the  efficiency  of  the  colonial  organism  is 
purchased  at  the  price  of  the  individual 
independence  of  the  units  which  compose  it. 
If,  however,  we  ask  the  question,  What 
advantage  do  the  cells  gain  by  this  union  ? 
the  answer  is  not  easy  to  give.  The  uni- 
cellular forms  succeed  very  well,  and  they  live 
in  the  same  sort  of  environment  as  the  multi- 


56  PLANT   LIFE 

cellular  colonies.  This  proves  at  once  that 
each  is  suited  for  existence,  in  so  far  as  physical 
conditions  are  concerned.  We  may  indeed 
inquire  whether  the  more  specialised  colonies 
actually  do  succeed  better  at  all  than  their 
simpler  unicellular  relatives.  In  the  higher 
forms  there  is  the  accumulation  of  food- 
supplies,  and  such  consequent  advantages 
as  accrue  from  the  possession  of  these  re- 
serves, but  it  is  clearly  impossible  to  see  how 
this  could  account  for  the  origin  of  the  multi- 
cellular  types.  Perhaps,  indeed,  we  are 
attacking  the  problem  at  the  wrong  end  by 
regarding  it  as  one  of  profit  and  loss  at  all. 
It  seems  at  least  as  likely  that  the  same  sort 
of  influence  which  we  discern  to  subsist 
between,  and  to  determine  the  organisation 
of  the  units  of  a  specialised  colony,  operates 
in  a  similar,  albeit  in  a  simpler  and  cruder,  way 
between  the  potentially  free  omits  of  a  primi- 
tive colony.  In  other  words,  the  cause  of 
coherence  is  primarily  independent  of  ad- 
vantage or  disadvantage,  and  may  hardly 
exceed  an  almost  accidental  lack  of  disunion 
(e.g.  in  Spirogyra);  or  it  may  depend  upon 
some  attractive  influence  which  causes  the 
units,  primarily  separate,  to  cohere  in  clusters, 
as  happens  in  the  series  of  algae  exemplified 
by  well-known  forms  such  as  Volvox  or 
Hydrodictyon. 


THE  GREEN  LEAF       57 


CHAPTER  VI 

THE   GREEN   LEAF 

WE  now  pass  from  the  study  of  the  lower 
types  of  green  plants  to  a  consideration  of  the 
higher  and  more  specialised  forms  of  terrestrial 
vegetation.  But  if  we  restrict  ourselves  to 
a  comparison  of  the  vegetative  organs  of  the 
more  highly  differentiated  algae  and  of  the 
higher  plants,  we  shall  be  struck,  not  so  much 
by  the  dissimilarities,  as  by  the  likenesses 
which  exist  between  them.  We  meet  with 
the  same  specialisation  of  the  shoot  into  a 
stem  bearing  thin  expanded  structures — 
the  leaves.  There  are  the  same  organs  for 
attaching  the  plant  to  rough  surfaces,  or 
anchoring  it  in  a  looser  sub-stratum.  It  is 
not  difficult  to  discern  in  the  influence  of  light 
the  common  factor  which  has  been  chiefly 
concerned  in  the  production  of  these  resem- 
blances, so  far  at  least  as  external  form  is 
concerned. 

But  when  we  probe  more  deeply  into  the 
matter  the  real  differences  between  the  two 
classes  of  plants  begin  to  make  themselves 
apparent.  They  consist,  so  far  as  the  vegeta- 
tive structure  is  concerned,  in  a  specialisation 
of  cells  on  the  part  of  the  land  plant  which 
may  reach  a  grade  of  complexity  almost 


58  PLANT  LIFE 

infinitely  beyond  that  to  be  encountered 
in  any  alga.  Furthermore,  the  organs  of 
attachment  in  the  land  plant  no  longer  serve 
merely  as  "  holdfasts,"  but  they  discharge 
important  functions  in  connection  with  the 
absorption  of  water  and  mineral  food-supplies. 
Their  structure  becomes  increasingly  modified 
with  reference  to  the  larger  functions  they 
have  to  discharge. 

In  another  respect,  also,  the  higher  plants 
differ  from  the  lower,  namely  in  the  greater 
degree  of  definiteness  with  which  their 
various  organs  are  produced.  In  other  words, 
the  organisation  of  the  individual  is  as  a  whole 
more  specialised,  and  is  less  apt  than  are 
simpler  types  to  vary  its  normal  sequences 
of  growth.  The  different  morphological 
structures  are  less  and  less  susceptible  of 
alteration  than  is  the  case  with  more  primitive 
plants,  in  which  the  bonds  of  correlation  and 
co-ordination  between  the  constituent  cells 
and  tissues  are  weaker. 

If  we  ask  why  there  should  be  this  advanced 
degree  of  anatomical  differentiation  associated 
with  a  land  habitat,  we  shall  find  the  answer 
to  lie  on  the  one  hand  chiefly  in  the  needs 
for  adequate  water  supply  and  all  that  this 
involves,  and  on  the  other  in  the  demand  for 
a  body  constructed  on  sound  mechanical 
principles,  so  that  it  may  be  enabled  success- 
fully to  withstand  the  various  stresses  and 
strains  to  which  it  is  continually  liable  to  be 
subjected. 


THE  GREEN  LEAF  59 

Neither  of  these  needs  is  specially  pressing 
in  the  case  of  water  plants,  and  indeed  we  find 
that  when  any  of  the  descendants  of  the  land 
flora  take  to  an  aquatic  life,  they  tend  more 
or  less  rapidly  to  lose  those  distinctive  ana- 
tomical characters  that  marked  their  terres- 
trial forebears.  Plants  which  are  growing 
submerged  in  water  are  obviously  better 
fitted  to  absorb  it  through  any  part  of  their 
surface  and  consequently  have  less  need  for 
elaborately  specialised  organs,  either  for 
absorption  or  conduction,  than  those  whose 
roots  alone  are  in  contact  with  the  damp  soil, 
while  the  rest  of  the  body  is  exposed  to  the 
drying  influence  of  currents  of  air. 

At  the  same  time  the  water  plants  also 
escape  most  of  the  mechanical  difficulties, 
and  easily  maintain  a  properly  spread  out  leaf 
surface,  and  even  an  upright  position,  owing 
to  the  circumstance  that  their  specific  gravity 
is  so  nearly  identical  with  that  of  water, 
Their  weight  thus  becomes  an  almost  negli- 
gible factor,  especially  as  they  are  often 
buoyed  up  in  the  water,  owing  to  the 
presence  of  air  or  gases  entangled  in  their 
tissues. 

But  most  of  the  higher  water  plants  have 
not  entirely  lost  the  traces  of  their  terrestrial 
inheritance.  Even  the  roots  of  many  of  them 
still  function  as  absorbing  organs,  and  the 
mechanical  tissue  is  often  present,  though  in  a 
more  or  less  rudimentary  condition.  Some- 
times, indeed,  as  in  species  that  inhabit 


60  PLANT  LIFE 

swiftly  flowing  streams,  the  roots  may  ex- 
hibit new  and  remarkable  developments  that 
especially  fit  their  possessors  to  occupy  such 
stations. 

Let  us  inquire  somewhat  more  closely  as 
to  what  are  the  special  qualities,  both  of 
general  behaviour  and  anatomical  structure, 
which  render  a  terrestrial  life  possible  for 
plants.  If  we  select  a  concrete  example  of 
a  land  plant,  such  as  an  oak  tree,  we  observe 
that  there  is  a  large  branching  top,  covered 
with  leaves  for  part  of  the  year.  Below,  this 
crown  passes  into  the  trunk,  and  the  latter 
again  ends  in  the  branched  root  system  under- 
ground. The  leaves  are,  of  course,  the  fac- 
tories in  which  the  operation  of  food-making 
is  going  on  so  long  as  they  are  exposed  to  the 
light.  The  roots  are  absorbing  water  from 
the  soil,  and  such  salts  as  are  dissolved  in  it, 
whilst  the  trunk  forms  an  intermediate  con- 
ducting region  through  which  exchange  be- 
tween the  substances  in  the  root  and  the  rest 
of  the  tree  can  take  place.  The  circulation 
of  materials  in  a  plant  is  not  really  like  the 
circulation  of  the  blood  in  animals,  although 
an  analogy — largely  a  false  one — is  often 
drawn  between  them,  for  there  is  no  con- 
tinuous circulating  system  in  the  oak  tree 
at  all  comparable  with  the  arteries  and  veins 
of  the  animal  body.  Nevertheless  there  is 
a  process  of  exchange,  though  arranged  on 
different  lines,  and  serving  quite  different 
ends.  In  order  to  grasp  this  clearly,  it  will 


THE  GREEN  LEAF 


61 


be  convenient  in  the  first  place  to  cast  a  brief 
glance  at  the  functions  of  the  leaf. 

The  leaf  absorbs  from  the  air  chiefly  oxygen 
ami  carbon  dioxide.  The  latter  gas  is  present 
in  very  small  quantities  only,  say  about  2-5 


Fig.  9. — Section  of  a  Leaf,  showing  the  internal  structure 
and  also  the  surface  of  the  lower  epidermis  (E) ;  C,  cuticle ; 
E,  E  epidermis ;  P,  palisade  cells  containing  much  chloro- 
phyll ;  S.M.,  the  more  spongy  lower  tissue  of  the  leaf  with 
abundant  air  spaces;  S,  stomata;  V,  B,  "  vein  "  or  vas- 
cular bundle,  consisting  of  wood  (W),  and  bast  (B). 

to  3  parts  in  10,000.  Yet  this  carbon  dioxide 
represents  the  sole  source  of  the  carbon  which 
forms  so  large  a  part  of  the  dry  weight  of  the 
tree.  Free  oxygen  is  required,  as  it  is  by 
almost  all  living  beings,  for  purposes  of 
respiration ;  that  is  to  say  for  the  purpose  of 
oxidising  certain  chemical  substances  within 


62  PLANT  LIFE 

the  cells.  This  property  of  respiration  secures, 
inter  alia,  an  economic  transformation  of 
energy  within  the  organism. 

In  respect,  then,  of  the  functions  of  respira- 
tion and  photosynthesis  an  oak  leaf  does  not 
primarily  differ,  in  essential  respects,  from  a 
seaweed.  But  in  the  important  matter  of 
water  relations  the  two  are  on  a  very  different 
footing.  It  has  already  been  pointed  out 
that  a  supply  of  water  to  the  living  cells  is 
essential  for  the  exercise  of  their  functions. 
The  alga,  in  its  watery  habitat,  has  no  diffi- 
culty in  this  respect,  but  the  oak  leaf,  so  far 
from  obtaining,  is  continually  losing  water 
from  its  surfaces.  Even  in  wet  weather  very 
little,  if  any,  of  the  rain  which  falls  on  it  is 
absorbed  by  the  cells.  This  is  owing  to  the 
circumstance  that  the  outer  layer  of  the  wall 
of  the  external  sheet  of  cells  (epidermis)  has 
undergone  a  change,  and  no  longer  consists 
of  cellulose,  through  which  water  can  readily 
pass.  It  has  become  converted  into  cuticle, 
which  is  extremely  impervious  to  water, 
and  partially  so  to  gases  as  well.  This 
cuticle  is  of  extreme  importance  to  terrestrial 
plants,  inasmuch  as  it  provides  one  of  the 
chief  means  for  preventing  their  losing  water 
by  the  ordinary  process  of  evaporation.  All 
the  water  required  by  the  leaf  is  received 
from  the  root  by  way  of  the  stem,  and  it  is 
distributed  to  all  parts  of  the  leaf  by  means 
of  the  vascular  bundles,  which  are  often  known 
as  the  "  veins "  of  the  leaf.  It  is  these 


THE  GREEN  LEAF  63 

"  veins  "  which  are  left,  and  constitute  the 
delicate  network  of  "  skeleton  leaves "  on 
macerating  leaves  in  water.  The  vascular 
bundles  are  rather  complicated  in  structure, 
and  they  represent  the  most  highly  specialised 
tissues  of  the  plant  body.  A  vascular 
bundle  consists  of  wood  (xylem)  and  bast 
(phloem),  and  a  thin  band  of  tissue  known 
as  cambium  often  lies  between  them.  They 
anastomose  freely  in  the  oak  leaf,  and  in 
the  stalk  they  are  collected  into  a  few  large 
vascular  strands  which  join  the  vascular 
tissues  of  the  stem.  Similarly  the  root  pos- 
sesses vascular  strands,  and  these  are  like- 
wise joined  with  those  of  the  stem  or  trunk, 
and  thus  there  is  a  general  continuity  of  the 
vascular  tissue  throughout  the  plant.  The 
water  enters  the  root  from  the  soil,  passes  up 
the  trunk,  and  flows  thence  into  the  leaves, 
travelling  through  certain  specialised  cell 
elements  of  the  wood.  In  the  leaf  it  is  dis- 
tributed to  various  kinds  of  cells,  and  especially 
to  those  containing  the  bulk  of  the  chloro- 
phyll (P,  in  Fig.  9)  in  which  photosynthesis 
is  especially  active.  The  greater  part  of  it 
evaporates  from  the  cells  into  the  large  air 
spaces  which  are  present  in  the  leaf  substance, 
and  the  contained  air  is  thus  saturated  with 
aqueous  vapour. 

The  cuticle,  which  forms  the  outermost 
membrane  of  the  epidermis,  would  prevent 
the  exit  of  any  water,  either  as  liquid  or 
vapour,  if  it  were  perfectly  continuous,  In  the 


64  PLANT  LIFE 

same  way  it  would  preclude  the  entrance  oi 
oxygen  and  carbon  dioxide,  at  any  rate  in 
sufficient  quantity.  But  as  a  matter  of  fact 
it  is  not  continuous.  There  are  immense 
numbers  of  minute  gaps  in  the  epidermis, 
termed  stomata  (Fig.  9,  S),  and  these  form 
the  external  orifices  of  an  extensive  system  of 
air  spaces  which  are  present  between  the  cells 
of  which  the  leaf  is  composed.  These  inter- 
cellular spaces  are  of  the  utmost  importance 
to  the  leaf,  inasmuch  as  it  is  by  means  of  them 
that  gaseous  exchange  between  the  cells  and 
the  atmosphere  is  rendered  possible. 

Each  pore  or  stoma  is  really  a  slit  formed 
between  two  sausage-shaped  cells  of  the 
epidermis,  and  these  two  guard  cells,  as  they 
are  called,  can  change  their  shape  according 
as  they  become  more  or  less  distended  with 
water.  When  they  are  distended,  or  turgid, 
the  aperture  between  them  becomes  wider,  as 
they  lose  water  the  pore  tends  to  close.  We 
see  then  that  the  leaf,  as  regards  water,  is  a 
beautifully  self-regulated  mechanism.  When 
a  plentiful  supply  is  available  the  opening  of 
the  stomata  enables  the  vapour  which  satu- 
rates the  air  in  the  intercellular  spaces  to 
diffuse  out ;  but  when  the  supplies  fall  short 
the  loss  is  avoided  by  the  closing  together  of 
the  guard  cells.  Other  things  being  equal,  it 
is  advantageous  that  water  should  be  abun- 
dantly available,  as  in  this  way  mineral  salts 
are  brought  to  the  leaves.  A  relatively  rapid 
flow  to  these  organs,  however,  only  takes  place 


THE  GREEN  LEAF  65 

when  the  surplus  vapour  is  constantly  passing 
away  through  the  stomata. 

But  the  stomata  are  important  as  the  means 
of  gaseous  intake,  as  well  as  for  the  output  of 
water  vapour  and  other  gases.  Now,  although 
the  apertures  are  very  numerous,  the  total  sum 
of  their  areas  reckoned  as  a  fraction  of  the 
surface  of  the  leaf  is  still  very  small.  The 
amount  of  carbon  dioxide  in  the  air  is  likewise 
very  minute,  and  yet  the  intake  of  carbon 
dioxide  is  very  large.  For  many  years  the 
explanation  of  this  apparent  anomaly  re- 
mained obscure,  but  investigations  revealed 
the  fact  that  the  leaf  actually  absorbs  as  much 
carbon  dioxide  as  if  its  chlorophyll-containing 
cells  were  exposed  freely  to  air,  and  were  not 
covered  by  a  membrane  or  epidermis  at  all. 
The  explanation  is  to  be  found  in  a  remark- 
able modification  of  the  ordinary  conditions 
of  diffusion  through  their  perforated  mem- 
branes. It  is  to  the  effect  that  when  the 
orifices  become  small  enough  the  rate  of 
diffusion  through  them  increases,  area  for 
area,  up  to  certain  limits.  Or  to  express  it 
more  precisely;  while  the  rate  for  relatively 
large  holes  varies  very  nearly  as  the  areas  of 
the  holes,  it  varies  as  the  diameters  of  small 
holes  if  these  are  sufficiently  spaced  apart. 

In  these  respects,  then,  the  leaf  is  an  organ 
admirably  adapted  for  the  discharge,  in  the 
most  efficient  manner  possible,  of  the  im- 
portant function  of  photosynthesis.  The 
necessary  passage  of  gases  and  water  vapour, 
E 


66  PLANT  LIFE 

whether  into  or  out  of  its  interior,  is  achieved 
as  the  result  of  a  nice  adjustment  to  the 
physical  conditions  that  regulate  the  diffusion 
of  gases  through  a  perforate  membrane.  If 
we  try  to  explain  to  ourselves  how  such  a 
mechanism  could  have  become  so  perfectly 
evolved,  how  the  correlation  between  the  cells 
of  the  epidermal  tissue  became  so  perfectly 
— and  apparently  so  purposefully — arranged 
and  adjusted,  we  shall  find  ourselves  con- 
fronted with  a  task  of  no  mean  order.  And 
the  same  difficulty  arises  whenever  we  attempt 
to  give  a  satisfactory  explanation  of  any 
other  instance  of  complex  adaptedness  in  the 
structure  of  living  things. 

Utilising  the  physical  advantages  which 
the  arrangement  of  its  constituent  cells  and 
tissues  have  placed  at  its  disposal,  the  oak 
leaf,  under  the  influence  of  light  from  the 
sun,  of  carbon  dioxide  from  the  air,  and  of 
water  from  the  soil,  carries  on  the  operation 
of  photosynthesis  in  certain  cells  which  are 
situated  just  beneath  the  epidermis.  From 
their  form  these  are  commonly  known  as 
"  palisade  cells,"  and  they  are  continuously 
active,  provided  the  general  conditions,  such 
as  suitable  temperature,  light,  and  adequate 
supplies  of  oxygen  and  of  carbon  dioxide, 
are  fulfilled.  The  need  of  oxygen  by  plants, 
in  contrast  to  animals,  is  a  very  modest  one, 
and  indeed  the  oxygen  which  is  liberated 
within  the  leaf  during  the  process  of  photo- 
synthesis may  really  suffice  for  respiratory 


THE  GREEN  LEAF  67 

purposes.  Sugar  then  begins  to  form  in  the 
manufacturing  cells.  But  it  is  a  character- 
istic feature  of  this,  as  of  so  many  other 
chemical  reactions,  whether  in  the  living  cell 
or  in  a  test-tube,  that  the  rate  of  formation 
of  the  soluble  product  slows  down  as  the  con- 
centration of  that  product  increases.  Any 
such  wasteful  lowering  of  the  rate  of  produc- 
tion is  avoided  in  the  plant  cell  by  the 
starting  of  a  second  process,  whereby  insolu- 
ble starch  is  formed  as  soon  as  the  con- 
centration of  the  sugar  in  the  cell  reaches  a 
certain  point.  The  sugar  is  thus  continually 
prevented  from  accumulating  in  quantities 
sufficient  to  bring  about  the  cessation  of 
photosynthetic  activity  within  the  cell. 

As  long  as  the  leaf  remains  attached  to  the 
tree,  a  certain  amount  of  the  sugar  is,  in  any 
event,  being  withdrawn  from  the  cells  in 
which  it  is  being  manufactured.  This  sugar 
does  not,  however,  diffuse  from  cell  to  cell 
in  any  casual  direction.  Thus  it  does  not 
readily  pass  from  one  palisade  cell  to  its 
adjacent  neighbour.  But  it  does  very  readily 
pass  into  the  subjacent  cells,  and  through 
them  to  the  vascular  strands  of  the  leaf. 
These  strands  consist,  as  already  explained, 
of  wood  and  bast  (or  xylem  and  phloem) 
and  it  is  mainly  through  the  cells  of  the 
latter  that  the  sugar  travels,  diffusing 
from  one  cell  to  another.  The  cells  of  the 
phloem  are  of  various  shapes,  but  they  are 
mostly  elongated  in  the  direction  of  the 


68  PLANT  LIFE 

strand,  and  some  have  the  transverse  walls 
which  separate  the  elongated  cells  of  a  row 
perforated  by  small  pores.  These  are  the 
sieve  tubes,  and  much  of  the  various  food 
substances  which  reach  the  vascular  strands 
passes  through  them.  But  it  is  probable  that 
such  an  easily  diffusible  substance  as  sugar 
passes  as  well  through  tracts  of  other  elon- 
gated, but  not  so  obviously  perforated,  cells 
of  the  phloem.  Be  this  as  it  may,  it  is 
largely  through  the  vascular  strands  that  the 
sugars  of  the  plant  are  carried  away  from 
the  regions  where  they  are  present  in  excess 
to  other  regions  where  they  are  relatively 
deficient.  This  occurs  whether  the  deficiency 
arises  through  the  sugar  being  directly  used 
up  in  the  chemical  operations  of  the  cells, 
or  whether  the  special  conditions  of  the  local 
deposition  of  food  reserves  are  such  as  to 
produce  a  diffusion  gradient,  that  is  a  steady 
flow  within  the  plant  from  a  place  of  high  to 
one  of  lower  concentration.  It  is  well  to 
emphasise  the  limitation  thus  expressed  in 
the  last  sentence,  for  however  readily  sub- 
stances may  travel  from  one  plant  cell  to 
another,  it  is  a  very  different  thing  if  one 
endeavours  to  get  them  to  diffuse  out  of  the 
region  of  the  living  cells  into  a  mass  of  sur- 
rounding water,  for  example.  Such  attempts 
commonly  do  not  succeed  unless  the  cell 
protoplasm  be  first  modified,  as,  for  example, 
by  means  of  an  anaesthetic  or  by  some  more 
violent  and  lethal  agent. 


THE  GREEN  LEAF  69 

If  a  leaf  which  has  been  active  enough 
to  have  accumulated  starch  in  its  tissues  be 
examined  after  a  sufficient  interval  during 
which  photosynthesis  has  been  in  abeyance 
(owing  to  the  absence  of  light,  for  example), 
the  amount  of  starch  will  be  found  to  be 
lessened,  and  it  may  have  all  disappeared. 
The  reason  of  this  lies  in  an  extension  of 
the  process  already  sketched  in  outline.  The 
sugar  continues  to  be  withdrawn  from  the 
leaf  cell  even  after  all  further  synthesis  has 
ceased.  But  as  the  concentration  of  the 
sugar  sinks,  a  ferment  action  makes  itself 
felt  within  the  cell.  The  starch  is  gradually 
attacked  by  a  ferment  or  enzyme  known 
as  diastase,  and  it  is  thus  converted  into 
a  soluble  sugar  called  maltose;  the  maltose 
then  continues  to  pass  away  from  the  cell,  or 
at  least  so  much  of  it  as  is  not  immediately 
required  by  the  cell  protoplasm  itself.  The 
process  of  migration  continues  till  all  the 
starch  has  been  fermented  and  rendered 
soluble. 

The  change  from  starch  to  sugar  is  a  very 
simple  one,  merely  involving  a  dislocation  of 
the  larger  molecular  aggregate  together  with 
the  incorporation  of  a  molecule  of  water.  It 
is  of  a  totally  different  order  of  change  to  that 
which  is  involved  in  the  oxidation  of  the 
carbohydrate.  For  oxidation  involves  a  con- 
siderable change  in  the  state  of  energy,  as 
well  as  of  chemical  constitution. 

The  leaf  starch,  thus  fermented  into  soluble 


70  PLANT  LIFE 

and  diffusible  sugar,  travels  in  the  latter  form 
to  other  parts  of  the  plant.  It  passes  to  the 
growing  regions,  where  it  is  utilised  in  growth 
processes,  to  storage  tissues  where  it  is  re- 
converted into  starch  or  into  some  other  food 
reserve,  or  it  is  drawn  towards  any  other  centre 
of  activity  where  a  consumption  of  carbo- 
hydrate is  in  progress. 

We  have  learned  in  a  former  chapter  that 
water  plays  an  important  part  in  photo- 
synthetic  production  of  carbohydrate.  It 
not  only  acts  as  a  physical  agent,  by  main- 
taining the  protoplasm  in  that  state  of  watery 
consistence  essential  to  chemical  change,  but 
it  also  forms  part  of  the  raw  material  which 
enters  into  the  actual  composition  of  the 
sugars  and  similar  substances.  Furthermore  it 
serves  as  the  vehicle  by  which  salts  containing 
phosphorus,  sulphur  and  other  substances 
which  enter  into  the  composition  of  proto- 
plasm, or  are  essential  to  its  proper  working, 
can  enter  the  plant  from  without.  The  excess 
of  water  is  eliminated  from  the  plant  by  the 
diffusion  or  transpiration  of  the  watery  vapour 
through  the  stomata. 


ROOTS  AND  THEIR  FUNCTIONS    71 


CHAPTER  VII 

ROOTS   AND   THEIR   FUNCTIONS 

IN  order  to  complete  our  story  of  the  green 
leaf  and  its  duties  to  the  plant,  we  must  know 
how  the  water  is  absorbed  into  the  plant  and 
how  it  is  transmitted  to  the  leaves  or  other 
organs  where  it  is  required. 

We  might  still  keep  the  oak  tree  before  us 
as  a  concrete  example  in  which  to  study  these 
things,  and  we  should  discover  that  it  is  only 
by  the  roots  that  the  tree  obtains  the  water 
it  needs,  and  that  these  organs  absorb  it 
directly  from  the  soil  in  which  the  tree  is 
growing. 

If  we  attempted  to  pull  the  roots  out  of  the 
ground  it  would  be  found  that,  even  in  a 
seedling  tree,  the  task  is  not  an  easy  one. 
They  penetrate  the  soil  deeply,  and  ramify 
widely  through  it.  It  is  easier,  therefore,  and 
for  certain  other  reasons  better,  to  study  the 
roots  of  a  more  easily  accessible  object — say 
a  sunflower  or  any  other  herbaceous  plant. 

On  carefully  digging  out  the  roots  of  such 
a  plant,  we  should  see  that  the  tips  are 
smooth  and  conical,  a  shape  well  suited  to  bore 
through  the  soil.  At  a  short  distance  behind  the 
tip,  the  root  is  rather  velvety  or  hairy,  and  it 


72  PLANT  LIFE 

is  impossible  completely  to  wash  the  soil  away 
from  this  portion.  Still  further  behind,  the 
soil  ceases  to  adhere  to  the  surface.  Other 
roots  or  rootlets  are  seen  to  be  growing  out, 
and  still  further  away  from  the  tip  the 
diameter  of  the  young  root  begins  evidently 
to  increase.  Thus  we  distinguish  four  regions  : 
(1)  the  tip  and  clean  surface;  (2)  the  hairy 
zone ;  (3)  the  region  from  which  young  rootlets 
are  springing ;  (4)  The  older  parts  which  are 
getting  thicker. 

The  only  part  of  the  root  which  is  actively 
absorbing  water  from  the  soil  is  the  hairy 
zone,  and  the  hairs  themselves — outgrowths 
from  the  superficial  cell  layer — are  the  essential 
structures  which  perform  this  task.  The  apex 
is  chiefly  concerned  with  boring  on  through 
the  soil,  and  it  is  covered  with  a  characteristic 
covering  of  cells  called  the  root-cap,  the  outer 
cells  of  which  are  continually  being  worn  away 
by  attrition  in  the  soil,  and  as  constantly  being 
replaced  by  the  formation  of  fresh  layers  from 
within.  The  superficial  cells  behind  the 
region  of  the  root-cap  do  not  begin  to  elongate 
at  once  to  form  hairs.  This  does  not  happen 
till  the  part  of  the  root  from  which  they 
spring  has  ceased  to  elongate.  The  meaning 
of  this  at  once  becomes  clear  when  we  reflect 
that  these  delicate  protuberances,  the  root- 
hairs,  are  in  very  intimate  contact  with  the 
particles  of  soil — and  if  the  part  of  the  root 
which  bears  them  were  to  continue  to  grow 
in  length,  they  would  be  torn  away  from  their 


ROOTS  AND  THEIR  FUNCTIONS    73 

attachment  to  the  soil.  They  are  only  efficient 
so  long  as  they  are  uninjured,  and  perhaps 
this  helps  to  explain  why  the  hairy  zone  is 
such  a  short  one  on  any  one  root,  for  the  hairs 
do  not  grow  again  when  once  they  are  injured 
or  worn  out.  The  underground  system  as  a 
whole,  however,  repairs  this  defect  by  forming 
a  mass  of  branching  roots,  each  one  of  which 
may  repeat  the  form  and  the  four  stages 
indicated  above. 

In  order  to  understand  how  the  root-hair, 
and  the  root  as  a  whole,  play  their  respective 
parts  in  the  absorption  of  water,  some  ac- 
quaintance with  the  cellular  structures  con- 
cerned is  necessary. 

We  can  ascertain  this  by  examining  under 
the  microscope  sections  of  roots  cut  in  various 
directions.  The  annexed  illustration  (Fig.  10) 
represents,  rather  diagrammatically,  a  trans- 
verse section  of  a  root.  The  hairy  outgrowths 
are  the  root- hairs.  They  consist  of  an  outer 
cell  wall  enclosing  the  living  protoplasm  which 
lines  the  interior  of  the  wall,  though  it  does 
not  fill  the  entire  space,  for  its  own  interior 
is  occupied  by  a  "  vacuole  "  of  watery  sap. 
Passing  inwards  from  the  superficial  root-hair 
layer  we  notice  a  band  of  "  cortical  "  cells 
consisting  of  several  layers  forming  the  rind. 
Still  more  interiorly  we  arrive  at  a  starlike 
arrangement  of  certain  cell  groups.  This 
inner  cylinder  is  the  vascular  strand  of  the 
root,  and  it  consists  of  wood  (xylem)  and  bast 
(phloem),  just  as  in  the  strands  of  the  leaf  or 


Fig.    10.-~Root   in   transverse   section.     B,  bast;   P,  pith; 
R,  rind  or  cortex;  W,  wood;  H,  root  hair. 


ROOTS  AND  THEIR  FUNCTIONS    75 

stem.  But  in  the  young  root  the  xylem  and 
phloem  are  arranged  alternately,  whilst  in  the 
stem  and  leaf  they  are  superposed  in  pairs, 
with  the  phloem  usually  exterior  in  position. 
The  cells  of  which  the  woody  or  xylem 
portion  of  the  vascular  strands  is  composed 
generally  undergo  a  peculiar  change  in  chemi- 
cal and  physical  characters  known  as  lignifi- 
cation.  Lignified  walls  are  less  extensible 
and  less  collapsible,  and  in  general  are  more 
rigid  than  the  ordinary  cellulose  membranes. 
Moreover  the  lignified  walls  often  become 
considerably  thickened,  which  further  empha- 
sises the  same  qualities. 

In  dealing  with  wood,  especially  in  the 
stem,  we  must  remember  that  we  are  con- 
cerned with  a  complicated  mass  of  tissues 
associated  with  the  discharge  of  many  and 
very  different  functions  (Fig.  11).  Some  of  the 
wood  tissues  are  concerned  with  storage  of 
food,  others  have  to  do  with  the  mechanical 
functions  of  support,  etc.  To  these  we  shall 
return  later,  but  the  special  tissues  of  xylem 
that  just  now  concern  us  are  those  which 
are  connected  with  the  conduction  of  water. 
The  cells  of  the  water-conducting  tissue  differ 
amongst  themselves  in  details,  but  they  are 
commonly  elongated  in  form,  and  are  arranged 
more  or  less  in  longitudinal  continuity.  It 
sometimes  happens  that  the  end  walls  separa- 
ting two  or  more  cells  become  perforated  or 
even  obliterated,  so  that  the  cavity  of  one 


76 


PLANT  LIFE 


cell  becomes  directly  continuous  with  those 
of  longitudinally  adjacent  cells.  Such  tubes, 
which  have  arisen  by  the  disappearance  of 


B 


Fig.  11. — Diagrammatic  section  of  young  wood  stem.  B,  B, 
bast ;  C,  C,  cambium ;  E,  epidermis ;  M,  later  wood ;  P, 
the  first  wood  formed  (protoxylem) ;  Pi,  pith;  B,  rind; 
S,  stoma;  V,  V,  vessel;  WP,  wood  parenchyma. 

walls,  are  often  called  vessels.  The  water- 
conducting  cells  which  keep  their  walls  intact 
are  termed  tracheids.  The  tracheids  and 
vessels  then  form  the  special  tissues  in  the 


ROOTS  AND  THEIR  FUNCTIONS    77 

xylem  which  are  concerned  with  translocation 
of  water.  The' walls  are  thickened,  but  nearly 
always  show  thin  spots  or  "  pits."  These  are 
of  special  use  inasmuch  as  the  water  from 
one  tracheid  can  more  easily  and  rapidly  pass 
through  a  thin  than  a  thick  membrane.  Now 
there  are  considerable  variations  of  pressure 
conditions  in  these  conducting  channels,  and 
an  unprotected  thin  membrane  would  stand 
a  good  chance  of  becoming  ruptured.  The 
risk  is  obviated  by  a  partial  roofing  over  the 
thin  spots  by  the  thickened  parts  of  the  walls, 
which  gives  the  pits  a  curious  appearance 
under  the  microscope,  and  has  caused  them 
to  be  known  generally  as  "  bordered  pits." 
Pits  of  this  kind  are,  as  we  might  now  antici- 
pate, of  almost  universal  occurrence  in  water- 
conducting  tissue.  They  are  more  easily  seen 
in  some  woods  than  others,  and  perhaps  in 
none  better  than  in  a  bit  of  deal  or  pine  wood 
(Fig.  12).  ^ 

A  striking  character  of  these  conducting 
tracheids  and  vessels  lies  in  the  absence  of 
living  protoplasm  from  them.  All  functional 
tracheids  and  vessels  are  therefore  merely 
the  dead  skeletons  of  once  living  cells.  The 
protoplasm  disappears  from  them  as  soon  as 
the  thickening  and  lignification  of  the  walls 
is  complete.  It  is  good  that  this  should  be 
so,  for  the  presence  of  viscous  protoplasm 
within  the  channels  would  greatly  impede  the 
flow  of  water  through  them. 

In  addition  to  the  conducting  tracheids  and 


78 


PLANT  LIFE 


vessels,  the  wood  always  contains  some  com- 
paratively undifferentiated  tissue  cells,  such 


'J 

(0 

T 

<0 


Fig.  12. — Diagram  to  show  the  bordered  pits  and  how  the 
tracheids  in  the  wood  of  a  pine  are  connected  with  each 
other.  LT,  lower  tracheid ;  UT,  UT,  two  upper  tracheids  ; 
BP,  bordered  pits.  At  the  top  a  bordered  pit  is  shown 
in  section  (very  highly  magnified). 

as   are    of    common    occurrence    throughout 
the  plant  body;   these,  from  their  ordinary 


ROOTS  AND  THEIR  FUNCTIONS    79 

form,  are  generally  called  parenchymatous 
cells  (Fig.  11,  WP).  The  wood  parenchyma, 
which  largely  serves  the  purpose  of  storage 
(or  occasionally  as  an  excretory  tissue)  is  very 
often  lignified.  There  are  also  other  cells 
which  are  specialised  for  mechanical  purposes. 
They  are  of  various  forms  and  sizes,  and  are 
grouped  into  more  or  less  definite  tissue 
systems.  To  the  consideration  of  the  latter 
we  shall  return  when  we  come  to  consider  the 
architecture  and  mechanics  of  the  plant.  For 
the  present,  however,  we  are  only  concerned 
with  those  tissues  of  the  wood  that  are  de- 
tailed for  the  service  of  translocation  of  water. 

The  vessels  and  tracheids  form  a  continuous 
communicating  system  in  the  plant,  and  when 
water  enters  this  system  it  can  readily  be 
transmitted  from  any  one  point  to  any  other, 
the  direction  of  flow  being  determined  by 
purely  physical  conditions  of  pressure. 

We  can  now  endeavour  to  trace  the  passage 
of  water  from  the  soil  into  the  water-conduct- 
ing tissue  of  the  plants,  and  thence  into  the 
leaves,  to  which  most  of  the  water  that  is 
absorbed  ultimately  finds  its  way.  The  root- 
hair  is  in  close  contact  with  the  particles  of 
soil,  and  it  not  only  absorbs  water  from  it, 
but  it  exerts  a  disintegrating  influence  on  it 
owing  to  the  excretion  of  carbonic  acid  from 
the  living  cell. 

The  absorption  of  water  (which  contains 
very  small  quantities  of  salts  in  solution)  by 
the  root-hairs  is  an  active  process,  and  it  has  to 


80  PLANT  LIFE 

work  in  opposition  to  the  surface-forces  that 
tend  to  retain  the  water  in  the  soil  as  a  film 
which  wets  the  minute  particles  of  which  soil 
is  composed.  The  root-hairs  are  very  closely 
adherent  to  some  of  these  particles,  and 
they  wrest  the  water  from  the  film  which 
surrounds  them.  This  disturbs  the  equili- 
brium of  the  water  as  distributed  in  the  soil, 
and  it  causes  a  constant  flow  towards  the 
spot  whence  it  is  being  abstracted.  It  is  to 
this  circumstance  that  much  of  the  drying 
effects  of  plants  on  soil  is  due,  for  the  total 
amount  of  root-hair  surface  of  a  tree  is  far 
smaller  than  the  area  of  ground  that  it  will 
drain.  Another  example  of  the  movements 
of  water  in  soil  is  seen  in  the  way  that  it  loses 
moisture  in  dry  weather.  This  is  because 
evaporation  is  going  on  at  the  surface  of  the 
ground,  and  water  is  continually  passing  up- 
wards from  the  lower  levels  to  replace  that 
which  has  passed  into  the  atmosphere  as 
vapour.  The  resistance  to  movements  of 
water  as  the  films  lining  the  soil  particles 
become  very  thin  rapidly  increases,  and  thus 
ordinary  ground  does  not  easily  become  dry 
for  a  great  distance  below  the  surface.  Any- 
thing that  disturbs  the  continuity  of  the  soil 
particles  also-  interposes  a  further  hindrance 
to  the  movement  of  water,  and  this  is  why  a 
garden  soil  that  is  kept  stirred  with  a  hoe 
withstands  drought  so  much  better  than  one 
that  is  not  cultivated  in  this  way.  The 
particles  of  soil  that  have  been  stirred  by  the 


ROOTS  AND   THEIR  FUNCTIONS     81 

hoe  are  separated  from  each  other,  and  thus 
the  continuity  of  their  surfaces  with  that  of 
the  lower  soil  is  largely  interrupted.  At  the 
same  time  the  broken,  loose  soil  serves  to 
check  evaporation,  inasmuch  as  it  shelters 
the  lower  unbroken  (therefore  continuous)  soil 
both  from  the  sun  and  from  the  drying 
influence  of  currents  of  air.  It  is  a  matter 
of  common  experience  that  if  plants  are 
grown  in  unwatered  soil  long  enough,  they 
begin  to  droop  and  wilt.  This  means  that 
the  root-hairs  are  not  able  to  extract  enough 
water  from  the  ground  to  keep  pace  with  that 
which  is  lost  by  the  plant.  Wilting  takes 
place  when  the  water  contents  of  the  soil  fall 
below  a  certain  amount,  and  this  varies  greatly 
in  different  soils,  but  is  fairly  constant  for 
each  particular  kind.  Thus,  in  sand  a  plant 
may  utilise  all  the  water  down  to  about  1-2%; 
while  in  heavy  clay  the  water  ceases  to  be 
available  as  soon  as  its  content  sinks  below 
about  25  %.  It  is  evident  that  there  is 
probably  some  relation  between  the  physical 
state  of  the  soil,  and  its  physiologically  avail- 
able water  content.  And  this  turns  out  to 
be  the  case.  The  fine  particles  of  clay,  with 
their  relatively  enormous  surface,  retain  far 
more  water  than  sand  with  its  large  particles 
and  relatively  small  surface.  Ingenious  ex- 
periments on  soil  in  centrifugal  machines  have 
shown  that  approximately  the  same  amount 
of  force  is  required  to  clear  out  water  from 
clay  so  as  to  leave  25  %  remaining  as  is 


82  PLANT  LIFE 

required  to  leave  about  1  %  in  coarse  sand. 
These  experiments  are  of  great  value  in 
enabling  us  to  see  the  way  to  attack  many 
problems  of  plant  and  soil  relations,  and  they 
show  that  the  notion  that  the  plant  has  forcibly 
to  wrest  the  water  from  the  soil  is  a  fairly 
accurate  one. 

How,  then,  does  the  root-hair  do  this  ? 
What  force  can  it  exercise  in  the  process  ? 

Experiments  show  that  a  plant  cell  will, 
in  general,  absorb  and  retain  water  with 
considerable  avidity.  This  it  does  by  means 
of  the  so-called  osmotic  pressure  exerted  within 
it  by  various  substances,  such  as  sugar, 
organic  acids,  and  the  like,  which  are  dissolved 
in  the  watery  sap  within  the  cell.  For  whereas 
water  can  pass  freely  in  and  out  of  the  cell, 
the  protoplasm  either  does  not  allow  the 
dissolved  substances  to  pass  out,  or  it  only 
lets  them  through  very  slowly.  Without 
going  at  all  fully  into  the  difficult  and  complex 
subject  of  osmotic  pressure  in  general,  it  may 
be  remarked  that,  under  these  circumstances, 
water  tends  to  flow  into  the  cell  and  to  such 
an  extent  that  the  cell  sap  exerts  a  very 
considerable  pressure.  This  may  easily 
reach  a  value  equivalent  to  about  eleven 
atmospheres.  It  is  this  circumstance  which 
at  least  partly  accounts  for  absorption  of 
water  from  even  relatively  dry  soil  by  the 
root-hairs,  to  make  good  that  which  is 
lost  from  other  parts  of  the  plant.  For,  as 
already  explained,  the  parts  of  the  plant  above 


ROOTS  AND  THEIR  FUNCTIONS    83 

ground,  and  especially  the  leaves,  are  contin- 
ually losing  water  through  the  stomata,  and 
the  water  in  the  xylem  conducting  cells  and 
vessels  is  being  as  continually  drawn  upon. 
Thus  the  whole  water  system  is  in  a  peculiar 
condition  of  considerable  "  tensile  stress." 
This  condition  may  be  compared  to  a  wire 
which  is  subjected  to  a  powerful  pull.  This 
comparison  may  appear  at  first  sight  to  be 
far  fetched,  but  it  really  does  illustrate  fairly 
well  what  is  going  on,  especially  in  the  water 
conduits  of  tall  trees.  For  when  water  is 
enclosed  in  suitable  tubes  (and  the  conducting 
tissues  of  the  water  conduits  are  suitable  in 
this  respect)  the  force  required  to  break  such 
a  column  of  water  is  very  great,  many  times 
that  of  the  pressure  of  one  atmosphere.  As 
everybody  knows,  in  an  ordinary  tube  water 
can  only  be  maintained  at  a  height  of  about 
32  feet  by  means  of  atmospheric  pressure 
alone.  But  pure  water,  completely  filling 
clean  tubes  of  appropriate  structure,  will 
maintain  itself  at  a  height  many  times 
32  feet,  owing  to  its  capacity  of  resisting 
tensile  stress. 

Although  there  are  certain  difficulties,  all 
of  which  have  not  as  yet  been  fully  met,  in 
explaining  the  movement  of  the  current  of 
water  up  through  the  trunks  of  tall  trees, 
there  is  little  doubt  that  the  principle  just 
indicated  is  the  main  factor  in  the  matter, 
for  though  the  column  of  water  thus  main- 
tained is  very  stable  as  a  whole,  the  individual 


84  PLANT  LIFE 

molecules  of  the  water  are  free  to  move  within 
the  column. 

Of  course,  this  condition  of  stress  is  propa- 
gated throughout  the  water  system  from  the 
leaf  back  to  the  root.  Water  thus  tends  to 
be  withdrawn  from  the  outer  cells  of  the 
root  which  abut  on  the  ends  of  the  conducting 
tissues  within,  and  in  this  way  a  continuous 
flow  is  maintained  from  the  root-hairs,  which 
in  their  turn  are  replenished  from  the  supplies 
of  water  contained  in  the  soil.  It  comes  to 
be  a  balance  of  forces  represented  on  the  one 
hand  by  those  leading  to  the  escape  of  water 
vapour  from  the  leaf,  and  on  the  other  the 
forces  which  tend  to  cause  the  water  to  be 
retained  by  the  soil  plus  the  effects  of  friction, 
etc.,  within  the  plant  itself. 

But,  as  a  matter  of  fact,  although  the  short 
description  given  above  probably  represents 
in  a  general  way  what  goes  on  in  connection 
with  the  translocation  of  water  in  a  plant,  there 
are  other  factors  which  are  involved  and  may 
affect  the  process. 

The  living  parenchymatous  cells  of  the 
roots  are  not  merely  passive  agents  in  the 
matter,  for  the  water  absorbed  from  the 
soil  is  in  many  plants  (and  perhaps  in  all) 
forcibly  pressed  or  excreted  from  these  living 
cells  into  the  conducting  channels.  It  is  to 
this  active  propulsion  of  water  within  the 
plant  that  the  phenomenon  of  "  bleeding  "  is 
due.  When  trees  are  felled  in  spring,  sap 
may  continue  for  a  long  time  to  flow  forcibly 


ROOTS  AND   THEIR  FUNCTIONS     85 

out  from  the  surface  of  the  wood.  If  vines 
are  pruned  too  late  the  water  thus  pressed 
out  by  the  root  cells  through  the  xylem  will 
flow  for  many  days,  and  it  is  squeezed  out 
at  a  pressure  often  amounting  to  several 
atmospheres.  The  maintenance  of  the  pres- 
sure depends  on  the  living  cells  of  the  root, 
hence  it  is  called  "  root  pressure."  Anything 
which  interferes  with  the  life  of  the  root  cells 
causes  the  pressure  to  diminish.  Thus  chil- 
ling the  roots,  depriving  them  of  oxygen,  or 
treating  them  with  anaesthetics  as  well  as 
with  other  poisons,  may  temporarily  or  per- 
manently abolish  root  pressure. 

No  very  satisfactory  explanation  has  been 
given  of  Toot  pressure,  nor  indeed  of  any  other 
form  of  excretion.  We  are  sure,  however,  that 
as  our  knowledge  of  the  physical  and  chemical 
processes  of  protoplasm  increases  the  diffi- 
culties will  one  day  vanish.  In  the  mean- 
time the  problems  connected  with  water 
absorption  and  its  movements  within  the 
plant  are  still  in  the  interesting  condition  of 
incomplete  solution.  We  know  more  or  less 
what  happens,  but  we  do  not  as  yet  fully 
understand  the  how  of  the  happening. 


86  PLANT  LIFE 


CHAPTER  VIII 

CORRELATION    OF   FUNCTION   AND   FORM 

IN  the  higher  and  more  specialised  green 
plants  the  organ  principally  charged  with 
carrying  on  the  important  function  of  photo- 
synthesis is  commonly,  though  not  invariably, 
the  leaf.  Now  a  green  leaf  only  "  pays  its 
way  "  for  so  long  as  it  is  adequately  exposed 
to  light.  But  the  intensity  of  light  which 
produces  the  best  results  varies  greatly  with 
different  plants.  Moreover,  a  plant  may  lose 
so  much  water  when  exposed  fully  to  the 
light,  and  hence  to  the  air,  that  any  advantage 
of  illumination  may  be  more  than  balanced 
by  the  chance  of  wilting.  We  find,  as  a 
matter  of  fact,  that  all  these  various  con- 
siderations are  of  practical  importance  in  the 
practice  of  forestry.  Some  kinds  of  trees 
will  tolerate  shade,  others  speedily  succumb 
if  they  are  not  fully  exposed  to  the  light. 
Beech,  for  example,  when  young  will  thrive 
under  the  shade  of  the  birch,  and  in  many 
places  it  is  best  raised  under  this  latter  tree, 
which  shields  it  in  various  ways  during  its 
infancy.  But  birch  will  not  grow  under 
beech.  One  is  a  strenuously  light-demanding 


FUNCTION  AND  FORM  87 

plant,  while  the  other  will  readily  tolerate 
shade. 

This  toleration  only  applies  to  the  plant 
taken  as  a  whole,  and  there  are  limits  beyond 
which  endurance  of  shade  does  not  go.  Most 
people  who  have  wandered  through  a  dense 
and  well-tended  wood  must  have  been  struck 
by  the  great  difference  between  the  forms  of 
the  individual  trees  which  compose  it  and 
those  of  the  same  species  grown  in  the  open 
or  in  the  hedgerow.  The  clean  tall  trunks 
and  the  compact  small  crown  of  the  forest 
tree  contrast  strongly  with  the  spreading 
growth  of  the  park  specimen.  And  yet  the 
difference  is  merely  a  consequence  of  the 
different  conditions  of  illumination.  A  tree 
grown  in  the  open  exposes  its  leaves  to  light 
on  all  sides.  The  spreading  limbs  space  out 
the  foliage  and  the  leaves  are  all  more  or  less 
actively  functional.  But  closer  inspection 
reveals  the  fact  that  it  is  only  the  leaves  on 
the  periphery  of  the  tree  and  its  branch 
systems  which  are  thus  flourishing.  The 
inner  portion  is  bare  of  leaves,  or  at  any  rate 
comparatively  so.  This  is  because  the  inner 
twigs  which  become  shaded  by  the  outer 
ones  are  starved,  and  sooner  or  later  they  die 
and  fall  off.  The  leaves  they  bore  did  not 
act  efficiently,  and  they  were  quietly  crushed 
out  of  existence. 

Precisely  the  same  thing,  on  a  different 
scale,  happens  when  young  trees  are  grown 
close  together,  as  in  a  well -managed  forest. 


88  PLANT  LIFE 

The  lateral  branches,  which  in  isolated  trees 
spread  out  and  constitute  its  charm,  here 
compete  with  each  other,  and  all  are  over- 
shadowed by  the  topmost  branches  which 
alone  get  properly  illuminated.  Consequently 
the  lower  leaves  become  useless,  and  they, 
together  with  the  branches  which  bear  them, 
become  starved  and  are  destined  to  perish. 
The  tops  are  constantly  growing  higher,  while 
the  trunk  is  as  perpetually  being  denuded 
of  its  lower  lateral  branches.  In  this  way 
the  grand  boles  or  trunks  are  formed  which  the 
woodman  delights  to  see,  and  they  are  the 
distinguishing  features  of  forests  managed  with 
skill  and  intelligence.  The  forester's  aim  is 
always  secured,  broadly  speaking,  in  this 
way,  though  of  course  there  are  differences 
in  actual  treatment  depending  on  the  par- 
ticular kind  of  tree  or  association  of  trees 
it  is  desired  to  produce.  The  tall  trunks  are 
the  result  of  a  sort  of  natural  pruning,  brought 
about  by  growing  the  trees  at  the  correct 
distances  apart,  the  actual  distance  being 
regulated  by  the  size,  age,  and  other  conditions 
which  affect  the  growth  as  a  whole. 

The  amount  of  leaf  surface  is  not  only 
influenced  by  the  available  exposure  to  light, 
but  is  influenced  by  other  conditions  as  well. 
Thus  mechanical  requirements  need  to  be 
satisfied,  and  they  may  easily  limit  the  dimen- 
sions practically  attainable  by  the  green 
surface  as  a  whole.  A  leaf  too  weak  in  itself, 
or  too  feebly  supported  to  retain  a  suitable 


FUNCTION  AND  FORM  89 

position  as  regards  the  source  of  light,  would 
be  as  useless  as  a  heavily  shaded  one.  More- 
over, with  the  increase  of  the  leaves  the 
mechanical  requirements  vitally  affect  the 
whole  subsidiary  apparatus  of  the  plant.  The 
root  is  concerned  in  this  no  less  than  the  stem, 
for  the  leaf  depends  for  the  proper  discharge 
of  its  functions  on  an  adequate  degree  of 
fixity  on  the  part  of  the  plant  as  a  whole. 

Another  factor  which  materially  influences 
the  foliar  organs  of  a  plant,  lies  in  the 
water  supply,  for  if  this  be  deficient  or 
precarious,  the  leaf  area  must  either  be 
correspondingly  reduced,  or  there  must  be 
found  some  means  of  checking  the  loss  of 
water,  or  else  the  difficulty  must  be  met  in 
yet  other  ways.  The  particular  form  of  solu- 
tion of  the  water  problem  which  happens  to 
be  adopted  by  any  given  plant  is  a  matter 
that  will  mainly  depend,  as  already  pointed 
out,  on  its  own  inherent  constitution. 

It  is  worth  while  to  endeavour  to  follow  out 
some  of  the  numerous  and  diverse  ways  by 
which  those  problems  relating  especially  to 
mechanical  needs  and  to  water  supply  have 
been  solved  by  various  sorts  of  plants.  Not 
only  shall  we  encounter  remarkable  examples 
of  adaptedness  to  special  conditions,  but  we 
shall  incidentally  be  brought  into  close  con- 
tact with  some  of  the  more  difficult  questions 
of  biological  philosophy. 

In  any  event  we  shall  gain  a  clearer  idea 
of  the  way  in  which  the  whole  anatomical 


90  PLANT  LIFE 

structure  and  indeed  the  whole  conformation 
of  the  plant  is  dominated  by  the  leaf  or  other 
equivalent  green  surface 


CHAPTER  IX 

MECHANICAL  PROBLEMS  AND  THEIR  SOLUTIONS 

WE  will,  in  the  first  place,  direct  our  atten- 
tion to  the  mechanical  problems  which  affect 
plants.  These  are,  broadly  speaking,  the 
same  as  those  which  confront  the  engineer 
in  his  ordinary  work  of  building  and  con- 
struction. There  are  a  variety  of  stresses 
and  strains  that  have  to  be  guarded  against, 
unless  the  fabric  is  to  collapse  either  by  its 
own  weight  or  by  the  action  of  other  external 
forces.  These  mechanical  requirements  are 
satisfied  in  practice  by  choosing  materials 
which,  in  the  first  place,  possess  the  requisite 
physical  characters  of  strength,  toughness 
and  the  like ;  and  in  the  second,  by  utilising 
them  to  the  best  mechanical  advantage 
economy  is  combined  with  efficiency. 

Now  it  may  safely  be  said  that  in  the 


MECHANICAL  PROBLEMS          91 

matter  of  engineering  construction  the  plant 
has  nothing  to  learn  of  man — although  the 
converse  might  not  be  equally  true.  The 
more  closely  one  examines  the  construction 
of  a  plant  from  the  mechanical  point  of  view, 
the  more  wonderful  and  complete  does  it 
appear.  Certain  cells  or  cell  tissues  become 
differentiated  from  their  neighbours,  and 
develop  the  requisite  strength,  elasticity, 
and  other  desirable  qualities.  They  are  not 
distributed  in  the  plant  at  haphazard,  but 
occur  in  situations  where  they  are  mechanic- 
ally effective  and  physiologically  appropriate. 
Furthermore,  they  are  united  so  as  to  form 
definite  tissue  systems,  and  in  connection 
with  the  more  specialised  types  we  may,  with- 
out any  exaggeration,  speak  of  a  mechanical 
arrangement  of  tissues. 

Suitable  rigidity  is  secured  by  the  young 
undifferentiated  parts  of  plants  in  the  same 
way  as  in  the  more  primitive  ones,  namely 
by  the  pressure  of  the  watery  sap  contained 
within  the  cells.  This  confers  the  same 
sort  of  resilience  as  an  inflated  rubber  ball 
possesses,  and  it  amply  suffices  for  many 
aquatic  forms,  although  it  is  not  sufficient 
for  the  needs  of  land  plants  generally,  and 
only  serves  for  small  species  growing  under 
special  conditions.  An  ordinary  rooted  plant 
has  not  only  to  hold  itself  in  position,  but  it 
has  to  be  capable  of  withstanding  the  effects 
of  forces  that  are  repeatedly  acting  upon  it. 
Every  time  the  wind  blows,  demands  are 


92 


PLANT  LIFE 


made   on   the   whole   mechanical   system   of 
the  plant.     If  it  bends  to  the  wind  it  ought 


Fig.  13 A. — Diagram  of  transverse  section  of  part  of  a  young 
stem  of  sunflower.  B,  bast ;  C,  cambium ;  Col,  collenchyma ; 
E,  epidermis;  H,  hair;  P,  pith;  E,  rind  or  cortex;  Scl, 
sclerenchyma ;  W,  wood. 

to  recover  its  old  position  when  the  blast  is 
over.  Its  roots  should  be  able  to  withstand 
the  stress  imposed  on  them,  and  prevent  the 


MECHANICAL  PROBLEMS 


93 


tree  from  being  pulled  out  of  the  ground  or 
blown  over,  "whilst  its  leaves  must  successfully 
resist  tearing  and  the  many  other  disruptive 
influences  to  which  they  are  exposed. 


L.R 


FIG.  13s. — Diagram  of  transverse  section  of  part  of  an  old 
stem  of  sunflower.  C,  cambium ;  E,  epidermis ;  LP, 
hard  lignified  parenchyma ;  P,  pith ;  Scl:  sclerenchyma ; 
W,  wood. 

An  ordinary  herbaceous  stem,  like  that  of 
a  sunflower,  affords  an  excellent  illustration 
of  the  development  of  mechanical  tissue  and  of 
its  disposition  so  as  to  secure  the  maximum  of 
efficiency  with  the  least  expenditure  of  material 


94  PLANT  LIFE 

(Figs.  13A  and  B).  A  fairly  young  stem  cut 
across  and  examined  under  a  moderate  magni- 
fication shows  that  the  centre  is  occupied  by 
a  bulky  pith  around  which  are  seen  the  cut 
ends  of  the  conducting  strands — the  vascular 
bundles.  Just  outside  these  are  to  be  found 
the  cut  ends  of  small  rod-like  strands  of  tissue, 
the  sclerenchyma.  These  run  down  the  stem, 
following,  roughly,  the  course  of  the  vascular 
bundles.  They  are  connected  laterally  at 
intervals,  and  especially  at  a  node  where  a 
leaf  springs  from  the  stem.  Each  of  these 
sclerenchymatous  strands  consists  of  very 
much  elongated  cells  with  pointed  ends  that 
grow  and  insert  themselves  between  their 
neighbours  above  and  below,  thus  giving  the 
rod  or  strand  of  sclerenchyma  as  a  whole  a 
considerable  degree  of  tenacity. 

The  walls  of  the  cells  become  greatly 
thickened, -.and  the  rod  is  sharply  marked  off 
from  the  soft  tissues  of  the  rind  in  which  it 
is  embedded.  Regarded  from  the  point  of 
view  of  its  physical  properties  it  exhibits 
remarkable!  strength.  It  is  quite  elastic  even 
when  submitted  to  considerable  stress.  This 
means  that  within  certain  limits  it  can  be 
pulled  out  (i.  e.  elongated)  by  applying  a 
force,  and  when  this  is  withdrawn  it  will 
recover,  and  contract  to  its  former  length. 
In  this  respect  the  sclerenchymatous  strands 
of  many  plants  are  but  little  inferior  to  good 
steel,  and  a  strand  one  millimeter  in  cross 
section  will  stand  a  pull  of  about  twenty 


MECHANICAL  PROBLEMS          95 

kilograms,  and  still  spring  back  when  the 
weight  is  removed.  They  elongate  more 
than  a  steel  wire  would  do  under  the  same 
stress,  and  they  differ  in  another  respect  from 
steel,  in  that  they  break  if  loaded  only  a 
little  beyond  their  elastic  limits. 

To  understand  how  effective  a  system  of 
such  strands  really  is  in  enabling  the  stem 
to  withstand  bending  stresses,  or  to  recover 
its  original  position  when  the  force  (e.  g.  that 
of  the  wind)  is  withdrawn,  we  must  consider 
the  way  in  which  they  are  arranged,  and  what 
actually  happens  when  a  stem  is  made  to 
bend.  In  the  first  place  the  sclerenchymatous 
strands  form  a  tissue  system,  and  in  the  second 
place  the  strands  cannot  shift  from  their 
relative  positions,  being  prevented  from  doing 
so  by  the  surrounding  cells  of  the  stem  which 
occupy  the  space  between  them.  If  we  there- 
fore consider  the  condition  of  two  of  these 
strands  situated  on,  let  us  say,  the  east  and 
west  sides  of  the  stem  they:  may  together  be 
regarded  as  forming  a  girder,  the  relatively 
weak  tissue  of  the  stem  lying  in  the  east  and 
west  plane  forming  the  "  webbing  "  or  lattice- 
work of  the  girder.  Now  when  a  girder  of 
this  construction  is  bent,  the  concave  side 
is  shortened  or  squeezed,  while  the  convex 
side  is  lengthened  or  pulled.  The  intervening 
webbing  merely  serves  to  hold  the  two  bars  or 
flanges  in  their  relative  positions  and  is  itself 
subject  to  less  and  less  stress  the  nearer  the 
middle  line  between  the  two  flanges  is  reached. 


96  PLANT  LIFE 

Exactly  the  same  happens  in  the  plant. 
When  a  sunflower,  or  a  wheat  stem,  is  bent 
by  the  wind  the  tissue  on  the  outer  side  is 
stretched,  that  on  the  concave  side  is  com- 
pressed, and  it  is  easy  to  see  that  both  con- 
ditions tend  to  straighten  the  stem  again. 
Since  the  axis  of  the  stem  is  neither  pulled 
nor  compressed,  it  is  obvious  that  there 
would  be  no  advantage  in  placing  the  mechan- 
ical tissue  where  the  pith  is ;  the  further  away 
from  the  axis,  i.  e.  the  nearer  to  the  circum- 
ference, the  more  effective  it  becomes. 

But  the  wind  does  not  act  only  in  the  east- 
west  plane,  and  plants  are  apt  to  be  subjected 
to  stresses  from  any  and  all  sides.  Thus  the 
girder  systems  are  more  complex  in  their 
arrangement,  and  are  so  multiplied  as  to  be 
ready  and  meet  the  stress  from  whatever 
quarter  it  comes.  Moreover,  they  commonly 
receive  additional  rigidity  by  being  tied 
together,  in  a  tangential  direction  as  well  as 
transversely,  by  specially  strong  tissues,  at  the 
nodes. 

Now  it  is  evident  that  this  form  of  mechanical 
tissue  is  not  suited  for  all  the  conditions  that 
may  be  experienced  by  stems.  For  example, 
the  young  parts  are  often  elongating,  and 
sclerenchyma  is  far  too  little  extensible  to 
admit  of  this  growth.  On  examining  such 
a  growing  region  we  find  that  although  the 
sclerenchyma  strands  are  recognisable  there, 
they  are  not  yet  functional.  In  fact  the  cells 
which  compose  them  are  only  beginning  to 


MECHANICAL  PROBLEMS          97 

develop,  the  cell  walls  are  still  thin,  the 
strand  is  itself  still  growing,  and  has  not  as 
yet  developed  those  properties  which  will 
ultimately  render  it  so  valuable  from  the 
mechanical  point  of  view. 

But  just  beneath  the  outside  skin  or 
epidermis  we  may  see  that  the  cell  layers 
which  make  up  the  periphery  of  the  outer 
rind  or  cortex  are  characterised  by  cell  walls 
of  a  remarkable  form.  They  are  much 
thickened,  especially  at  the  corners  where 
the  cells  abut  on  each  other,  and  this  thicken- 
ing often  extends  to  the  tangential  walls, 
while  the  radial  ones  usually  remain  thin. 
The  general  impression  they  give  is  that  of 
a  number  of  concentrically  arranged  bands 
of  thick  substance  (=  the  tangential  walls) 
bound  together  by  thin  plates  (=  the  radial 
walls).  These  thickened  walls  possess  remark- 
able mechanical  qualities  which  are  very 
different  from  those  which  distinguish  the 
sclerenchyma.  They  are  much  weaker,  but 
this  is  partly  compensated  by  their  more 
advantageous  position  at  the  periphery  of 
the  stem.  The  essential  feature  in  which 
they  differ  from  sclerenchyma  lies  in  the 
ease  with  which  they  can  be  stretched  beyond 
the  elastic  limits,  for  a  weight  of  about  two 
kilograms  suffices  to  produce  a  permanent 
elongation  in  a  strand  of  one  millimeter  in 
cross  section.  They  differ  still  further  from 
sclerenchyma  in  that  they  do  not  break  at 
this  limit,  but  will  stand  a  much  stronger  pull, 
Q 


98  PLANT  LIFE 

by  which  they  can  be  very  greatly  lengthened, 
although  they  become,  of  course,  considerably 
thinner  as  the  result.  This  tissue  is  often 
called  collenchyma,  from  the  peculiarly  bright 
gelatinous  appearance  of  the  walls.  It  is 
specially  adapted,  by  its  extensibility,  to  the 
requirements  of  small  and  growing  organs, 
whilst  its  inferior  value  as  a  supporting  tissue 
is  largely  compensated  by  its  advantageous 
position  in  the  stem.  Indeed,  collenchyma 
affords  a  wonderful  example  of  an  accurate 
balance  of  qualities  possessed  by  a  tissue 
which  is  required  to  be  carefully  adjusted  to 
meet  very  diverse  needs.  For  whilst  the 
function  of  support  is  its  main  raison  d'&tre, 
it  is  obvious  that  it  must  not  be  so  strong 
or  so  rigid  as  to  materially  interfere  with  the 
growth  in  length  of  the  organ  in  which  it  is 
present. 

It  sometimes  happens  that  the  structures 
on  which  the  rigidity  of  a  stem  depends  have 
to  be  provided  in  a  rather  different  way.  In 
wheat,  and  most  other  grasses,  the  stem 
continues  for  some  time  to  elongate  just  above 
the  node  or  "  knot."  Most  people  know  that 
it  is  easy  to  pull  the  stem  out  from  the  knot, 
and  that  the  broken  end  is  soft  and  succulent. 
But  a  series  of  such  weak  joints  in  a  stem, 
however  well  the  mechanical  requirements 
might  be  fulfilled  in  the  intervening  regions, 
would  of  course  be  fatal  to  the  retention  of 
an  erect  position.  In  the  grass  this  weakness 
is  remedied  by  a  curious  arrangement  of  the 


MECHANICAL  PROBLEMS          99 

leaf,  which  at  first  sight  often  seems  to  spring 
from  the  stem  some  distance  above  the  node. 
In  reality,  however,  the  lower  part  of  the  leaf 
forms  a  cylindrical  sheath  surrounding  and 
supporting  the  succulent,  elongating  portion  of 
the  stem  just  above  the  actual  node.  The 
cylindrical  leaf  sheath  is  supplied  with  abun- 
dant sclerenchyma,  which  is  arranged  in  a 
more  complex  way  than  in  the  sunflower,  but 
again  in  the  strictest  accordance  with  what 
we  have  discovered  to  be  sound  mechanical 
principles. 

It  is  the  mechanical  tissue  which  forms 
the  economically  valuable  fibre  yielded  by 
many  plants — such  as  hemp,  flax,  jute  and 
the  like — and  for  commercial  purposes  it  has 
to  be  separated  by  various  processes  from  the 
softer  tissues  in  which  it  lies  imbedded. 

As  a  plant  becomes  larger,  the  crushing 
effect  of  the  increasing  weight  of  the  foliage 
and  branches  begins  to  make  special  demands 
for  additional  mechanical  tissue.  This  is 
most  often  provided  for  by  a  large  increase 
in  the  tissues  of  the  wood.  In  a  cross  section 
of  such  a  plant  as  an  old  sunflower  the  wood 
is  seen  to  have  assumed  the  form  of  a  hollow 
cylinder,  variously  buttressed  and  thickened 
towards  the  pith. 

In  many  of  the  perennial  plants  the  character 
of  the  mechanical  supporting  tissue  is  less 
obvious,  principally  because  it  has  to  serve 
several  purposes,  and  also  because  it  is  rela- 
tively so  abundant  that,  if  the  expression 


100  PLANT  LIFE 

may  be  allowed,  it  does  not  seem  to  matter 
much  how  it  is  disposed. 

Every  one  must  have  noticed  that  the  great 
majority  of  our  shrubs  and  branching  trees 
increase  in  girth  as  they  get  older.  This  in- 
crease is  produced  by  a  specially  active  layer 
of  "  embryonic  "  tissue  known  as  cambium 
(Fig.  lie),  which  forms  a  cylindrical  sheet  of 
cells  situated  at  the  outer  limit  of  the  wood, 
which  it  thus  completely  encloses.  By  the 
active  division  of  this  cambium  the  cylinder 
or  zone  of  young  cell  tissue  is  temporarily 
rendered  thicker  every  year,  and  then  the 
layers  of  cells  which  abut  on  the  existing 
wood  are  themselves  differentiated  into  xylem, 
to  form  the  new  annual  ring  of  wood  which 
is  added  every  year  to  the  wood  of  the  trunk. 
A  few  of  the  outermost  layers  of  the  cylinder 
are  similarly  transformed  into  bast  or  phloem, 
and  only  a  thin  cell  layer  now  remains  as 
a  cylindrical  sheet  of  cambium  which  still 
continues  to  separate  the  wood  and  bast. 
Next  year  this  again  increases  in  thickness, 
and  the  new  layers  thus  produced  go  through 
the  same  changes  as  before. 

In  this  way  the  annual  rings  of  wood  are 
produced  which  are  seen  when  tree  trunks 
are  sawn  across.  It  is  due  to  the  still  un- 
differentiated  and  relatively  thick  sheet  of 
young  cells  produced  every  spring  that  the 
"  bark "  is  so  easily  separated  from  the 
wood  at  this  season.  For  the  walls  are  thin 
and  the  cells  are  rich  in  protoplasm  and  cell 


MECHANICAL  PROBLEMS        101 

sap.  They  are  thus  easily  ruptured,  and 
every  country  boy  knows  that  in  late  spring 
the  bark  of  willow  or  ash  twigs  can  easily  be 
slipped  off  the  wood  as  an  unbroken  cylinder. 
This  is  because  the  debris  of  the  torn  young 
cambial  cells  serve  as  a  sort  of  lubricant 
which  facilitates  the  process.  Later  on  the 
bark  will  not  slip  off  readily  if  at  all,  and 
this  is  due  to  the  fact  that  the  inner  and  outer 
cell  layers  of  the  previously  undifferentiated 
zone  have  gradually  become  changed  into 
wood  and  bast.  The  thin  layer  of  residual 
cambium  is  now  not  thick  enough,  nor  can 
it  provide  sufficient  lubricant  to  enable  the 
ring  of  bark  to  slip  off. 

The  new  wood  thus  produced  consists  of 
young  water-conducting  tracheids  and  vessels, 
as  well  as  of  other  sorts  of  cells  which  have 
various  functions  to  discharge.  Some  of 
these  cells,  as  they  change  from  the  embryonic 
to  the  permanent  or  adult  state  not  only 
thicken  their  walls,  but  grow  considerably 
in  length,  inserting  their  tips  between  other 
similar  cells  above  and  below  them.  They 
are  largely,  though  not  exclusively,  of  mechan- 
ical significance.  From  a  commercial  point 
of  view  it  is  mainly  to  the  mechanical  tissue 
that  woods  of  various  sorts  owe  their  technical 
value  as  timber. 

A  relatively  considerable  proportion  of  the 
new  wood  is  thus  more  or  less  definitely 
differentiated  to  serve  mechanical  purposes. 
This  applies  to  most  of  the  cells  which  have 


102  PLANT  LIFE 

thick  and  lignified  walls,  whether  they  are 
specifically  mechanical,  or  are  discharging 
other  functions  as  well,  such  as  that  of 
storing  supplies  of  food  in  the  trunk.  An 
enormous  proportion  of  the  wood  of  ordinary 
trees  consists,  then,  of  thick -walled  cells  of 
various  kinds  which  are  more  or  less  intimately 
knit  together,  with  the  result  that  the  whole 
possesses  not  only  considerable  strength  but 
also  a  high  degree  of  resilience.  This  latter 
quality  differs  greatly  in  different  timbers, 
but  it  is  entirely  the  result  of  the  properties 
of  the  individual  cell  walls,  combined  with  the 
manner  in  which  the  cells  themselves  have 
interdigitated  with  one  another. 

An  ordinary  tree,  by  virtue  of  these  pro- 
perties of  the  wood,  is  able  to  withstand  the 
effects  of  a  direct  crushing  stress  far  greater 
than  it  will  ever  be  called  to  meet  in  nature. 
It  has  also,  by  virtue  of  its  resiliency,  the 
faculty  of  recovering  its  position  when  it  is 
swayed  or  bent  by  the  wind. 

As  regards  the  great  lateral  branches  of 
large  trees,  their  heavy  weight  of  foliage  and 
small  branchlets  renders  the  need  for  power 
of  resistence  and  recovery  from  strains  of 
various  sorts  even  more  pressing.  Some- 
times, indeed,  they  prove  inadequate,  as  when 
a  branch' becomes  overloaded  with  fruit.  In 
the  present  year  (1912)  the  great  weight  of 
beech  mast  is  causing  many  large  branches 
to  bend  down  till  they  have  come  to  rest 
upon  the  ground,  and  in  not  a  few  instances 


MECHANICAL  PROBLEMS        103 

large  arms  of  the  trees  have  snapped,  because 
the  mechanical  tissues  have  proved  inadequate 
to  meet  the  unusual  demands  thus  thrown 
upon  them. 

When  we  compare  the  mechanical  arrange- 
ments of  the  root  system  of  a  plant  with  those 
affecting  its  aerial  portions,  we  are  at  once 
confronted  by  a  new  set  of  factors.  There  are 
two  sets  of  conditions  which  largely  control 
and  limit  the  possible  lines  of  variation  in  the 
mechanical  structure  of  roots.  One  of  these 
concerns  the  apical  growth  of  the  organ  as 
it  burrows  through  the  soil,  the  other  relates 
to  the  pull  exerted  on  the  root  system  by 
the  swaying  of  the  parts  above  ground 
when  "  they  are  fretten  with  the  gusts  of 
heaven." 

As  regards  the  growing  points  of  the  roots, 
the  means  for  pushing  forward  in  the  soil  is 
at  the  same  time  extremely  simple  and  most 
effective.  Unlike  the  stem,  the  actually 
elongating  portion  of  the  root  is  situated 
a  very  short  distance  behind  the  conical 
apex,  and  lies  just  in  front  of  the  zone  of 
root-hairs  already  described  (p.  73).  The 
latter  affords  a  sort  of  support  which  holds 
this  part  of  the  root  immovable,  whilst  the 
turgid  cells  of  the  very  short  growing  region, 
as  they  expand  in  growth,  drive  the  smooth 
conical  tip  resistlessly  forward.  If  the  growing 
region  were  a  long  one,  as  it  is  in  the  stem, 
there  would  be  an  imminent  risk  of  buckling, 
as  may  be  easily  understood  if  we  consider 


104  PLANT  LIFE 

what  a  thin,  cord-like  organ  the  ordinary 
young  root  is. 

Farther  back  from  the  growing  point  the 
mechanical  function  of  the  root,  as  already 
stated,  is  that  of  holding  the  plant  in  the  soil. 
The  most  effective  position  for  the  mechanical 
tissue  to  occupy  to  withstand  pulls  from 
various  directions  is  along  the  centre  or  axis 
of  the  organ.  For  in  this  position  the  stress 
is  most  evenly  distributed.  Indeed,  the  me- 
chanical strands  may  be  regarded  as  cables 
in  this  form  of  construction. 

Sometimes  it  happens  that  a  root  has  to 
discharge  still  more  complex  mechanical 
functions,  and  its  structure  in  this  respect 
may  then  vary  accordingly.  The  Indian  corn 
plant,  for  example,  has  a  thick  stem,  large 
leaves,  and  heavy  fruit  (Fig.  14).  The  rooting 
end  where  it  penetrates  the  ground  is  quite 
thin,  and  the  plant  is  obviously  top-heavy, 
but  a  circle  of  roots  springs  from  each  of 
the  nodes  of  the  stem,  that  succeed  each  other 
at  very  short  intervals  just  above  the  level 
of  the  ground,  and  each  root  grows  towards 
the  soil  in  a  more  or  less  arched  manner; 
in  this  way  the  plant  as  a  whole  is  well 
supported  by  means  of  a  series  of  arched 
struts  which  admirably  enable  it  to  over- 
come the  mechanical  disadvantages  of  its 
original  conformation.  Now  it  is  clear  that 
when  the  plant  is  exposed  to  a  force  tending  to 
bend  it,  the  roots  on  the  side  towards  which 
it  inclines  to  fall  over  are  exposed  to  crushing 


I- — Lower  part  of 
stem  with  aerial  roots, 
the  circles  (c)  represent 
roots  which  have  been 
cut  away,  the  horizon- 
tal bar  represents  the 
soil  surface. 


II. — Transverse  section 
(diagrammatic)  of  the  aerial 
part  of  a  root,  showing  the 
peripheral  rinp  of  mechani- 
cal tissue  in  the  rind  P  M 
Internally  there  is  the  wood 
which  represents  the  in- 
ternal mechanical  tissue 
1  M  •  the  bast  or  phloem  B. 

III. — Do.  of  subterranean 
root  with  no  peripheral 
ring. 


B 


Fig.  14. — Indian  corn. 
105 


106  PLANT  LIFE 

or  bending  stress.  An  axial  cord  of  mechan- 
ical tissue,  however  strong,  would  be  useless 
to  resist  such  a  stress  on  the  lee  side,  while 
the  arched  form  of  the  roots  would  minimise 
the  value  of  an  axile  strand  of  a  root  on  the 
windward  side.  This  special  form  of  mechan- 
ical stress  is  overcome  by  the  Indian  corn 
roots  in  a  remarkable  way.  The  extra- 
terrestrial arched  parts  of  the  roots  have  a 
thick  ring  of  mechanical  tissue  specially 
differentiated  from  the  cells  of  the  outer  rind, 
whilst  at  the  same  time  they  retain  the  cord- 
like  axile  strand.  Thus  these  roots  are 
excellently  adapted  to  withstand  both  crush- 
and  pulling  strains  from  whichever  quarter 
they  may  come.  Beneath  the  ground  only 
the  pulling  strains,  of  course,  are  operative, 
and  we  find  that  the  peripheral  thick  tissue 
ring  is  not  formed  in  the  subterranean  parts 
of  the  root  system. 


CLIMBING  AND  WATER  PLANTS    107 


CHAPTER  X 

SPECIAL    FEATURES    OF    CLIMBING    AND 
WATER  PLANTS 

THERE  are  several  groups  of  plants  in 
which  the  stems  are  more  or  less  exposed  to 
forces  similar  to  those  commonly  affecting 
the  roots  we  have  been  considering.  We  find 
that  such  stems  often  exhibit  corresponding 
deviations  from  the  normal  stem  structure, 
whereby  they  become  enabled  to  withstand 
these  special  directions  of  stress. 

One  of  the  most  interesting  examples  is 
furnished  by  the  great  group  of  climbing 
plants.  The  climbers  have  sprung  from  non- 
climbing  ancestors,  in  the  different  ranks 
of  the  vegetable  kingdom,  but  we  here  are 
concerned  only  with  those  which  belong  to 
the  flowering  plants.  Many  obvious  points 
of  close  similarity  are  shared  by  all  climbers, 
however  distantly  related  they  may  be  in 
other  respects.  They  usually  possess  rela- 
tively thin  main  stems  which  are  dependent 
on  other  objects,  bushes,  trees  and  the  like, 
for  their  support.  By  growing  to  the  tops 
of  the  latter  they  are  enabled  to  expose  their 
own  abundant  foliage  freely  to  the  light 
without  the  economic  disadvantages  attendant 


108  PLANT  LIFE 

on  the  development  of  a  correspondingly 
thick  trunk. 

But  the  various  exigencies  and  risks  in- 
separable from  a  climbing  habit,  have  given 
free  scope  to  the  play  of  individual  variation 
among  the  numerous  species,  both  related 
and  unrelated  to  each  other,  of  which  the 
great  group  of  the  climbers  is  composed. 
It  is  this  circumstance  that  gives  them  their 
special  interest,  and  also  renders  them  so 
instructive. 

Many  of  the  climbers  which  grow  in  the 
tropical  jungles  exhibit  extreme  specialisation 
in  connection  with  their  climbing  habits,  by 
which  they  are  enabled  rapidly  to  reach  the 
leafy  canopy  of  the  forest,  although  this  is 
often  many  feet  above  the  ground.  Some- 
times they  steal  a  march  on  circumstances,  as 
it  were,  and  the  seed  is  able  to  germinate  in 
the  upper  fork  of  a  tree.  This  occurs  in 
many  of  the  large  figs,  e.  g.  the  India-rubber 
Fig,  which,  perhaps,  can  hardly  be  called 
a  climber  in  the  ordinary  sense  of  the  term. 
Plants  of  this  kind  produce  roots  which 
rapidly  grow  downwards  and  penetrate  the 
soil,  the  young  fig  securing  the  great  ad- 
vantage that,  when  its  foliage  sprouts  forth, 
it  is  very  soon  fully  exposed  to  light. 

Other  climbers  behave  differently,  and 
more  nearly  resemble  the  kind  of  growth  of 
an  ordinary  plant,  but  with  certain  significant 
differences.  The  seed  germinates  on  the 
ground,  and  the  thin  shoot,  which  grows 


CLIMBING  AND  WATER  PLANTS    109 

upwards,  along  or  through  the  supporting 
vegetation,  only  produces  minute  leaves,  and 
at  very  distant  intervals.  It  is  ,not  until  the 
roof  of  the  forest  is  reached  tnat  the  large 
crop  of  big  green  leaves  is  unfolded,  and  their 
weight  is  entirely  borne  by  the  vegetation 
over  which  they  are  growing  and  spreading. 
Entangled  as  the  climber  becomes  among  the 
branches  of  the  sustaining  trees,  it  is  evident 
that  when  the  latter  are  swayed  by  the  wind, 
the  danger  of  snapping  which  confronts  its 
thin  stems  is  a  very  real  one.  Furthermore, 
while  the  plant  is  a  young  one,  the  risk  of 
being  parted  from  the  root  is  not  small.  These 
difficulties  are  all  obviated  in  several  ways. 

In  many  of  them  the  first  formed  wood  of 
the  young  plant  consists  almost  entirely  of 
strong  mechanical  tissue,  and  this  is  especially 
true  of  those  climbers  which  produce  no 
functional  leaves  worth  mentioning  till  they 
reach  the  roof  of  the  jungle.  The  presence  of 
this  axile  cord  of  sclerenchymatous  wood  is 
most  important  to  all  these  plants,  for  they 
need  to  be  very  flexible,  and  at  the  same 
time  to  be  able  to  withstand  very  considerable 
pulls  which  might  otherwise  snap  them 
asunder.  The  fact  that  they  are  admirably 
constructed  in  these  respects  is  illustrated  by 
the  name  of  "  jungle  ropes,"  by  which  so 
many  of  them  are  commonly  known — a 
popular  tribute  to  their  flexibility  and  their 
very  great  strength. 

But  it  is  evident  that  stems  constructed 


110  PLANT  LIFE 

on  the  lines  just  indicated  have  other  functions 
besides  purely  mechanical  ones,  and  these 
must  be  adequately  discharged  if  the  plant 
is  to  be  a  success.  As  soon  as  the  foliage 
is  produced,  water,  is  imperatively  demanded, 
and  thus  there  arises,  so  to  speak,  a  conflict 
between  opposing  requirements.  The  scleren- 
chymatous  tissue  is  excellent  for  supplying 
the  needed  strength,  and  its  axile  position 
renders  it  very  effective.  But  it  is  of  little 
or  no  use  as  water-conducting  tissue.  Now 
as  a  matter  of  fact  we  find  that  in  the  higher 
types  of  climbers  (e.  g.  many  members  of  the 
natural  orders  Leguminosae,  Sapindaceae, 
Bignoniaceae,  etc.),  that  this  strong  flexible 
axile  core  is  succeeded  externally,  and  quite 
suddenly,  by  vessels  of  wide  calibre  which, 
though  admirable  as  water  conduits,  are 
practically  useless  regarded  from  the  stand- 
point of  material  strength.  But  the  latter 
defect  loses  all  significance  as  the  plant  grows 
older,  for  the  difficulties  that  were  to  the  fore 
in  the  climber's  earlier  life  become  obviated 
later  on  in  a  very  simple  manner.  If  one 
observes  an  old  climber  in  the  jungle,  the 
lower  part  of  the  stem  is  often  seen  to  be 
lying  in  snaky  coils  on  the  ground,  and  is 
evidently  not  at  all  exposed  to  any  serious 
tractive  forces.  The  peculiarity  in  question 
is  due  to  the  fact  that  up  above,  in  the  roof 
of  the  forest,  the  lower  leafy  branches  of 
the  climber  are  dying  back  as  they  give  place 
to  the  younger  ones  springing  nearer  the 


CLIMBING  AND  WATER  PLANTS    111 

growing  points.  Consequently  the  stem  is 
gradually  falling  downwards  as  the  older 
anchoring  branches  die  and  rot  away.  A 
further  cause  of  the  same  slackening  of  the 
stem  is  to  be  discovered  in  some  climbers, 
depending  on  the  odd  circumstance  that  the 
growth  in  length  of  the  stem  continues  long 
after  it  would  '  have  ceased  in  ordinary 
plants. 

The  general  effect  of  this  elongation  of  the 
stem  below  the  forest  roof,  in  whatever  way 
it  is  produced,  is  to  relieve  it  entirely  of  all 
danger  from  tensile  stress.  Hence  the  stem 
can  now,  to  the  great  advantage  of  the  plant, 
become  almost  entirely  concerned  in  provid- 
ing the  means  for  the  transmission  of  water 
from  the  roots  to  the  mass  of  foliage  above. 
A  secondary  consequence  also  is  to  be  seen 
in  the  development  of  the  other  tissues  by 
which  the  food  material  manufactured  by 
this  foliage  is  distributed  in  the  plant.  If 
much  of  it  is  withdrawn  to  the  roots  the  stem 
is  rich  in  phloem,  but  it  is  not  especially 
so  if,  as  is  generally  the  case,  most  of  the 
manufactured  food  is  immediately  utilised 
in  the  copious  production  of  flowers  and  fruit. 

The  structure  of  such  specialised  climbers 
as  these  is  capable  of  being  interpreted  as 
the  result  of  a  compromise,  so  to  speak, 
between  the  opposing  functions  of  nutrition 
and  mechanics.  The  compromise  is  more 
obvious  than  in  the  majority  of  land  plants 
because  the  issues  are  more  strictly  defined. 


112  PLANT  LIFE 

The  narrow  diameter  of  the  stem  is  incom- 
patible with  waste  or  inefficiency  in  any  of 
its  parts,  whilst  by  its  unsuitability  to  act 
as  an  organ  of  storage,  the  obscuring  effects 
of  subsidiary  functions  are  comparatively 
eliminated. 

But  there  is  always  a  danger  in  an  appeal 
to  metaphor,  and  the  suggestion  conveyed  in 
the  term  compromise  ought  not  to  be  accepted 
as  containing  any  definite  explanation  of  the 
facts,  for  it  implicitly  begs  the  whole  ques- 
tion as  to  whether  the  plant  can  adapt  itself 
to  the  exigencies  of  a  particular  environment ; 
it  rather  indicates,  without  actually  giving,  an 
affirmative  reply.  But  it  may  well  be  that 
the  question  is  to  be  answered  in  a  totally 
different  way,  and  that  what  strikes  us  at 
first  sight  as  an  obvious  "  adaptation  "  may 
be  still  better  described  as  an  "  adaptedness  " 
brought  about  by  causes  and  conditions  not 
at  all  directly  connected  with  the  circum- 
stances under  which  they  are  so  clearly 
appropriate.  In  other  words,  the  power  of 
direct  adaptation  may  be  (and  probably  is) 
a  very  small  part  of  the  whole  problem  of  the 
fitness  so  generally  to  be  discerned  between 
the  plant  (or  animal)  and  its  natural 
surroundings. 

It  is  a  remarkable  circumstance  that  many 
of  the  climbers,  especially  the  more  advanced 
ones,  exhibit  a  considerable  degree  of  anoma- 
lous structure  in  their  stems,  and  especially 
in  their  main  stem.  A  large  number  of 


CLIMBING  AND  WATER  PLANTS    113 

these  anomalies  are  of  obvious  advantage 
to  a  climber,  and  are  calculated  to  minimise 
risk  of  damage  to  the  conducting  channels 
of  the  stem  under  the  special  circumstances 
of  their  habit  of  life.  The  main  stem  is 
sometimes  lobed,  and  it  may  ultimately  even 
split  into  a  rope-like  mass  of  cordage.  Or 
it  may  be  flattened  and  wavy  in  contour, 
a  character  obviously  associated  with  con- 
siderable resilience.  Again,  the  soft  phloem 
is  frequently  embedded  amongst  the  woody 
tissues,  and  is  thus  shielded  from  injury 
such  as  might  arise  through  torsion  of  the 
stem,  and  in  other  ways. 

But  it  is  not  true  that  every  specialised 
climber  is  provided  with  a  special  or  anomalous 
stem  structure,  nor  are  these  abnormalities 
confined  to  climbing  plants.  The  facts  seem 
to  indicate  that  the  anomalies  in  question 
are  to  be  regarded  as  instances  of  a  break 
away  from  traditional  structure,  that  they 
owe  their  origin  primarily  at  least  to  the 
inner  constitution  of  the  living  substance  of 
the  plants  in  which  they  arise.  They  may 
be  regarded  as  one  of  the  expressions  of 
inherent  tendency  to  vary  which  in  dominant 
groups  of  plants  is  seen  in  a  multiplication 
of  related  species.  Any  such  break  away 
from  the  type  form  of  structure  may  prove 
useful  in  enabling  a  plant  to  develop  new 
functions,  or  more  perfectly  to  discharge 
nascent  ones.  And  there  are  a  very  large 
number  of  instances,  of  the  most  varied 


Fig.  15. — Bauhinia  anguina,  with  special  hooked  branches 
which  help  the  plant  to  climb. 


114 


CLIMBING  AND  WATER  PLANTS    115 

kind,  which  indicate  that  when  an  organism 
has  once  modified  its  constitution  so  as  to 
exhibit  any  special  trend,  the  chances  are 
all  in  favour  of  advance  along  the  new  lines, 
and  very  slightly  indeed  in  favour  of  a  return 
to  the  old  ones.  The  history  of  abortion  of 
parts  (e.  g.  leaves),  of  concrescence  in  flowers, 
and  many  other  morphological  series  of  facts, 
may  be  adduced  in  support  of  this  proposition. 

Thus  while  stem  anomaly  is  often  (but 
far  from  invariably)  associated  with  climbing 
habit,  the  connection  is  seen  to  be,  after  all, 
rather  obscure.  Sometimes  perhaps  fortui- 
tous, at  others  it  is  to  be  regarded  as  the 
independent,  but  concomitant,  and  mutually 
advantageous  result  of  a  modification  of 
the  living  substance  of  the  plant  itself. 
Finally,  it  is  not  unlikely  that  the  abnormali- 
ties are  sometimes  elicited  as  the  response, 
on  the  part  of  plants  which  have  the  faculty 
of  making  them  at  all,  to  stimuli  given  to  the 
living  cells  by  the  strains  and  torsions,  as  well 
as  by  the  internal  nutritive  conditions  specially 
characteristic  of  the  climber,  or  incident  to 
the  climbing  habit. 

The  same  sort  of  argument  may  be  extended 
to  apply  to  the  well-known  fact  that  in  many 
climbers  certain  definite  organs  become  modi- 
fied, and  are  enabled  thereby  to  attach  the 
plant  to  a  support.  The  particular  organ 
(Figs.  15  and  16)  affected  varies  widely  in 
different  plants,  but  whether  it  is  a  hook,  a 
branch,  a  leaf,  or  part  of  a  leaf,  it  is  constant 


Fig.  16. — An  old  Bauhinia  stem ;  one  of  the  hooked  branches, 
or  tendrils,  has  grasped  a  support,  while  the  other,  which 
has  not  become  functionally  useful,  has  remained  thin. 


116 


CLIMBING  AND  WATER  PLANTS    117 

for  the  individual  species.  The  most  special- 
ised of  these  organs  are  the  tendrils. 

Tendrils  may  be  formed  from  specially 
modified  branches  as  in  the  passion  flower, 
or  from  leaf -stalks  as  in  clematis,  or  from  the 
leaves  or  leaflets  as  in  various  species  of 
vetch,  and  even  from  roots  as  in  the  vanilla 
orchid.  But  they  all  tend  to  become  very 
similar  in  form,  and  to  assume  in  common  just 
those  characters  that  enable  them  so  well 
to  discharge  their  functions. 

But  the  very  fact  of  their  diverse  origin 
(from  leaves,  stems,  etc.)  in  the  different 
plants  suffices  to  emphasise  the  importance 
of  what  we  may  call  the  internal  living 
factor,  as  opposed  to  the  environment  directly, 
in  their  production.  And  this  is  further 
strengthened  by  the  circumstance  that  they 
are  produced  fully  formed,  they  are  not 
gradually  and  tentatively  produced  and 
perfected  during  their  development,  any  more 
than  are  any  of  the  historically  older  organs 
of  the  plant.  But  nevertheless,  many  of 
them  are  endowed  with  the  faculty  of  further 
growth  in  thickness  and  strength  if  they 
become  functionally  active.  This  power  is 
not  restricted  to  tendrils  but  is  of  widespread 
occurrence,  and  is  especially  obvious  in  the 
case  of  the  stalks  of  heavy  fruits.  These, 
like  functional  tendrils,  greatly  increase  the 
amount  of  mechanical  tissue  primarily  present 
in  their  tissues  as  the  fruits  increase  in  weight. 
The  advantage  secured  is  in  both  examples 


118  PLANT  LIFE 

the  same,  but  the  real  causes  responsible  for 
its  appearance  are  equally  obscure.  The  most 
we  can  at  present  say  is  that  in  the  exercise 
of  the  function  new  conditions  are  introduced 
which  lead  to  the  supply  of  abundant  nutritive 
material,  together  with  the  power  to  use  it. 
But  the  mode  of  interaction  of  all  the  inner 
functional  conditions  is  far  too  complex  for 
us  to  express  the  matter  in  any  rough-and- 
ready  formula.  Least  of  all  is  it  useful  to  say, 
in  anthropomorphic  fashion,  that  structural 
peculiarities  like  those  of  climbers  are  due  to 
the  plant  having  adapted  itself  to  its  environ- 
ment. We  readily  discern  that  the  plant  is 
adapted,  but  we  know  remarkably  little  about 
the  processes  whereby  this  interrelation  has 
been  brought  about.  It  conduces  neither  to 
clearness  of  judgment  nor  to  the  advance 
of  science  to  mistake  more  or  less  fanciful 
descriptions  for  real  explanations  of  complex 
phenomena. 

Groups  of  plants  such  as  climbers  are 
interesting  for  the  very  reason  that  they  serve 
to  illustrate  the  fact  that  any  species,  what- 
ever its  ancestral  origin,  may  join  a  specialised 
biological  class  provided  it  has  the  capacity 
for  developing  an  appropriate  structure. 
Another  biological  group  is  constituted  by 
the  higher  water  plants.  These  have,  for  the 
most  part,  descended  from  terrestrial  fore- 
bears, and  they  display  many  significant 
features  of  interest  in  connection  with  their 
more  recent  environmental  conditions.  Some 


CLIMBING  AND  WATER  PLANTS    119 

of  them  are  still  amphibious,  and  are  able 
to  respond  to  the  stimulus  of  either  land 
or  watery  surroundings  by  a  suitable  struc- 
ture. For  example,  the  common  Moneywort 


Fig.  17. — Stem  of  Watei  Milfoil  (Myriophyllum)  in  transverse 
section.     A,  airspace;  VS,  vascular  strand. 

(Lysimachia  Nummularia)  of  wet  meadows 
will  grow  as  an  aquatic,  a  marsh  plant,  or 
as  an  inhabitant  of  dry  soil.  The  reason 
of  this  is  to  be  sought  in  the  way  in  which 
the  development  of  the  cuticle  is  affected, 


120  PLANT  LIFE 

though  only  indirectly,  by  the  environment, 
whereby  the  loss  of  water  is  limited  when 
the  moneywort  is  flourishing  in  dry  soil. 
Most  plants  do  not  share  this  faculty  of 
quickly  altering  their  chemical  processes  so 
as  to  become  adapted  to  so  wide  a  range  of 
conditions. 

The  most  striking  character  common  to  all 
the  higher  water  plants  consists  in  the  enormous 
development  of  intercellular  spaces  (Fig.  17). 
These  air-spaces,  communicating  finally  with 
the  atmosphere  by  the  stomata,  represent 
an  exaggerated  development  of  an  aerating 
system  that  occurs  in  every  land  plant.  The 
aquatics  have  not,  in  this  respect,  acquired 
anything  new,  they  have  merely  enlarged 
and  often  specialised,  what  was  already  an 
ancestral  trait.  Such  an  aerating  system 
sharply  marks  off  the  higher  water  plants 
from  the  lower  ones.  In  the  larger  seaweeds 
it  is  true  that  there  is  often  a  localised  forma- 
tion of  air  cavities.  These,  however,  serve 
rather  as  floating  organs  than  for  the 
general  purposes  of  respiration  and  gaseous 
exchange  generally. 

The  remarkable  congeries  of  trees  and 
shrubs  that  make  up  a  mangrove  swamp  in 
the  inlets  and  estuaries  on  tropical  coasts 
furnish  striking  examples  of  specialised  aerat- 
ing systems.  It  is  the  roots  which  run 
through  the  mud  of  the  swamp  that  are 
principally  affected  by  the  urgent  need  of 
free  oxygen.  After  the  root  of  a  mangrove 


CLIMBING  AND  WATER  PLANTS    121 

has  grown  for  a  certain  distance,  it  bends 
up  out  of  the  mud  into  the  air.  Then  as  it 
grows  on,  it  curls  down  and  its  tip  again 
enters  the  mud.  The  bowed  portion  or 
44  knee  "  which  sticks  up  into  the  air  forms 
a  quantity  of  spongy  tissue  full  of  intercellular 
spaces,  and  as  these  communicate  externally 
with  the  atmosphere,  and  internally  with 
the  intercellular  spaces  in  the  rest  of  the  root, 
the  respiration  of  the  root  cells  is  amply 
provided  for,  although  there  is  no  supply  of 
free  oxygen  in  the  mud  through  which  they 
grow. 

A  number  of  other  trees  of  the  mangrove 
swamp  form  special  roots  which  grow  up  like 
spikes  out  of  the  mud.  They  do  not  again 
turn  and  grow  downwards,  but  are  definitely 
specialised  as  aerating  organs.  They  may  be 
compared  to  ventilating  pipes,  for  their  use 
is  entirely  confined  to  enabling  an  interchange 
to  take  place  between  the  air  in  the  plant  and 
that  of  the  atmosphere. 

The  submerged  aquatic  plants  have  to 
meet  a  set  of  conditions  very  different  from 
those  which  confront  the  land  vegetation. 
Inasmuch  as  they  are  surrounded  with  water 
there  is  no  risk  of  desiccation,  and  the  cuticle 
is  poorly  developed  and  often  is  hardly 
perceptible.  Water  is  not  continually  being 
lost,  nor  is  there  any  difficulty  in  obtaining  it. 
Hence  it  is  not  surprising  to  find  that  the 
development  of  water-conducting  elements  is 
feeble.  And  one  of  the  striking  features  of 


122  PLANT  LIFE 

the  water  plants  consists  in  the  degenerate 
character  of  the  wood.  This  poverty  in 
water-conducting  tissue  is,  however,  chiefly 
to  be  seen  in  the  parts  which  have  elongated. 
The  nodes,  whence  the  leaves  arise,  often 
exhibit  quite  a  considerable  amount  of  vessels 
and  tracheids.  These  parts  of  the  stem, 
which  do  not  elongate,  are  in  sharp  contrast 
to  the  internodes,  in  which  practically  all  the 
growth  in  length  of  a  stem  takes  place.  In 
the  internodes  the  wood  is  often  merely 
rudimentary,  and  it  may  be  absent  alto- 
gether. On  the  other  hand,  the  phloem,  in 
which  the  organic  substances-  mainly  travel, 
is  usually  as  well  developed  as  in  a  land  plant, 
and  sometimes  even  better,  relatively  speaking. 
As  regards  the  mechanical  tissues,  aquatics 
are  specially  interesting.  They  are  ^almost 
of  the  same  specific  gravity  as  the  water,  and 
when  the  air-spaces  are  taken  into  account 
they  are  usually  much  lighter.  Consequently, 
arrangements  for  providing  the  mechanical 
condition  of  support  are  unnecessary  and 
would  be  wasteful.  The  only  serious  me- 
chanical requirements  are  those  adapted  to 
prevent  the  plants  growing  in  swift  torrents 
being  torn  asunder  by  the  force  of  the  current. 
We  find  that,  on  the  whole,  the  mechanical 
tissue,  when  present,  and  also  the  vascular 
strands,  tend  to  occupy  an  axile  position. 
This  is  especially  advantageous  for  the  latter, 
as  the  bending  and  waving  movement  of  the 
flexible  stems  will  naturally  cause  the  mini- 


CLIMBING  AND  WATER  PLANTS    123 

mum  of  distortion  in  vessels  and  cells  which 
lie  in  the  central  region  of  the  stem. 

As  far  as  aquatic  plants  are  concerned,  it 
is  only  for  those  which  inhabit  torrents  that 
the  mechanical  tissue  possesses  much  real 
significance.  In  the  ordinary  vegetation  of 
ponds  and  sluggish  rivers,  a  large  number 
of  the  stems  of  the  submerged  vegetation 
are  provided,  it  is  true,  with  strands  of 
mechanical  tissue,  but  they  often  appear  to 
be  scattered  rather  at  haphazard  through  the 
substance  of  the  stem  as  a  whole.  Indeed, 
it  almost  seems  as  if  these  water  plants  had 
rather  free  play  in  the  differentiation  of  the 
tissue  in  question.  Ordinary  aquatic  condi- 
tions are  not  constant  or  strenuous  enough 
to  demand  a  high  standard  of  mechanical 
efficiency.  Hence  the  less  rigorously  adapted 
individuals  are  not  eliminated,  and  the  average 
of  the  race  in  this  respect  is  soon  corre- 
spondingly lowered. 

We  may  complete  our  survey  of  mechanical 
tissues,  and  kindred  matters,  by  briefly 
considering  the  mode  of  construction  of  the 
leaf  from  this  point  of  view. 

The  flattened  shape  of  most  foliar  organs 
evidently  exposes  them  to  risks  of  being 
torn,  and  furthermore  it  is  of  prime  import- 
ance that  the  ordinary  leaf  should  retain 
an  extended  form,  and  not  easily  buckle; 
otherwise  the  chlorophyll  would  cease  to  be 
advantageously  displayed  to  light.  The 
danger  of  buckling  is  partly  met  by  the 


124  PLANT  LIFE 

thickness  and  resilience  of  the  epidermis,  but 
much  more  effectively  by  the  strengthening 
of  the  abundant  "  veins  "  or  vascular  bundles 
which  run  through  it.  These  form,  especially 
on  the  underside,  a  connected  system  of 
projecting  and  supporting  strands. 

In  the  elongated  strap-shaped  leaves  of 
grasses,  irises,  palms,  and  suchlike  plants,  in 
which  the  principal  veins  pursue  a  longitudinal 
course  in  the  leaf,  we  encounter  the  most 
beautiful  examples  of  precise  mechanical 
construction  by  which  the  proper  form  and 
position  of  the  leaf  is  maintained,  and  is  again 
recovered  after  any  displacement  that  may 
have  occurred.  Bands  of  sclerenchyma  run 
down  the  leaf,  just  below  the  upper  and  lower 
epidermis,  and  they  are  often  placed,  girder- 
wise,  opposite  one  another,  with  the  vein  or 
vascular  bundle  running  down  between  them. 
The  latter  thus  occupies  the  position  of  the 
webbing  of  a  girder.  Although  there  is  a 
good  deal  of  difference  in  the  details  of  differ- 
ent plants,  the  general  application  of  sound 
mechanical  principles  of  construction  and 
arrangement,  as  well  as  the  presence  of  suit- 
able strengthening  tissues,  is  patent  to  any 
observer  who  cares  to  examine  the  leaves. 

The  netted  veined  leaves  of  ordinary 
dicotyledons  are  exposed  to  considerable 
risks  of  damage  by  tearing  the  margins. 
The  forms  of  many  leaves  se^em  at  first  sight 
almost  to  invite  the  risk  of  tearing,  but  any 
one  who  tries  will  soon  convince  himself 


CLIMBING  AND  WATER  PLANTS     125 

that  it  is  only  an  apparent,  and  not  a  real 
risk.  The  fact  is,  the  veins  form  a  beautiful 
system  of  unions  or  anastomoses,  and  these 
are  often  arranged  in  a  series  of  arches  which 
pass  just  under  the  indentations  characteristic 
of  the  leaf  margins  of  so  many  plants. 

There  are  certain  exceptions,  however,  to 
the  general  rule  that  leaf  construction  is 
adapted  to  prevent  tearing.  All  palms  have 
leaves  which  are  primarily  undivided.  But 
by  complicated  processes  which  result  in  the 
dying  out  of  strips  of  leaf  tissue  extending 
from  the  midrib  to  the  margin,  the  leaf 
surface  as  a  whole  may  be  broken  up  into 
strips  resembling  pinnae  or  leaflets.  This 
occurs  in  the  Coco-nut,  and  many  other 
palms.  These  "  leaflets  "  are  very  different 
from  the  true  leaflets  of  a  vetch  and  most 
other  plants,  where  they  arise  as  the  result 
of  a  true  process  of  branching.  The  so-called 
fan-leaved  palms  have  leaves  in  which  there 
is  no  great  elongation  of  the  "  midrib,"  and 
the  pleated  or  concertina-like  folding  represent 
the  imperfect  separation  of  the  "  leaflet " 
which  is  only  completely  carried  out  in  forms 
like  the  Coco-nut,  Areca,  and  other  pinnate- 
leaved  species. 

The  Banana  plant  is  especially  interesting 
in  this  connection,  for  it  is  provided  by 
nature  with  a  leaf  quite  unsuitable  for  a 
plant  growing  in  any  but  the  most  sheltered 
situations.  The  banana  plant  consists  of  a 
thick  herbaceous  axis,  sheathed  by  the  bases 


126  PLANT  LIFE 

of  the  huge  leaves.  The  midrib  of  each  leaf 
is  a  massive  structure,  and  it  possesses  a 
considerable  degree  of  rigidity.  The  blade, 
which  it  traverses,  forms  a  long  oval  expansion, 
and  thus  exposes  to  the  air  a  very  considerable 
surface.  Any  one  acquainted  only  with  the 
banana  as  it  grows  in  a  plant  house,  where 
the  air  is  always  quiescent,  might  easily 
imagine  that  these  large  unbroken  leaves 
must  be  remarkably  well  provided  with 
mechanical  tissue  in  order  to  maintain  their 
outline  intact.  As  a  matter  of  fact,  however, 
precisely  the  reverse  is  the  case.  The  veins 
run  out  almost  at  right  angles  from  the 
midrib  to  the  margin,  and  anastomose  very 
little  with  each  other.  There  is  thus  no 
mechanical  reason  why  the  leaf  should  not 
be  easily  torn,  and  as  a  matter  of  fact  this 
is  what  actually  happens  to  a  plant  grown 
out  of  doors.  The  whole  blade  is  reduced  to 
a  number  of  separate  flaps  or  strips,  each 
firmly  attached,  of  course,  to  the  midrib. 
Hence  they  can  easily  give  to  the  breeze,  and 
the  banana  escapes  the  overthrow  to  which 
it  would  be  liable  were  it  to  hoist  such  large 
leaves,  if  unbreakable,  in  the  teeth  of  the 
wind.  The  efficiency  of  the  leaf  surface  is 
practically  unimpaired  by  the  tearing,  because 
the  vascular  bundles,  running  parallel  to  each 
other,  are  not  broken  across,  and  their 
functions  as  conducting  channels  to  and  from 
the  midrib  to  the  green  leaf  surface  are  not 
interfered  with  in  any  way. 


ADAPTATION  127 


CHAPTER  XI 

ADAPTATION 

THE  sketch  of  the  formation  and  distribu- 
tion of  mechanical  tissues  attempted  in  the 
last  few  chapters,  raises  rather  forcibly  the 
question  of  how  the  existence  and  elaboration 
of  the  green  leaf  has  succeeded  in  so  pro- 
foundly affecting,  even  in  this  one  particular, 
the  construction  of  the  whole  organism.  Of 
course,  we  recognise  that  the  influence  of  the 
leaf  depends  on  its  position  as  the  chief  bearer 
of  the  chlorophyll  of  the  plant,  and  to  this 
extent  our  question  becomes  more  precise. 
But  if  we  limit  ourselves  for  the  moment  to 
the  consideration  of  this  single  problem  of 
mechanical  adaptation  and  correlation,  in 
order  to  try  to  get  a  clear  issue,  we  find  that 
the  issue  is  far  from  being  clear,  and  the 
approaches  to  the  problem  itself  bristle  with 
difficulties. 

It  is  true  that  we  can  readily  find,  in  our 
analysis  of  the  influence  of  the  leaf,  a  very 
complete  justification  for  the  various  me- 
chanical adaptations  and  correlations  which 
we  have  learnt  to  recognise.  It  is  but  one 
aspect  of  the  much  larger  generalisation  that 
there  is  a  real  and  obvious  relation  between 


128  PLANT  LIFE 

the  structure  of  the  organism  and  its  environ- 
ment. It  is,  further,  almost  a  truism  to 
remark  that  the  more  complex  the  organism 
the  more  patent  is  the  perfection  of  its 
adjustment. 

It  is  only  when  we  get  at  closer  quarters 
with  our  problem  that  its  intricacies  really 
begin  to  reveal  themselves,  and  we  are 
obliged  to  confess  that  our  search  for  the 
causes,  and  even  for  the  proximate  agents  by 
which  the  production  of  appropriate  mechan- 
ical tissue  is  produced,  has  not  been  greatly 
rewarded.  Of  the  means  whereby  the  corre- 
lation is  secured  between  functional  need  on 
the  one  hand,  and  its  peculiarity  correct 
satisfaction  on  the  other,  we  have  no  positive 
knowledge  at  all. 

It  is  easy  to  talk  of  "  capacity  to  vary," 
"  survival  of  the  fittest,"  and  so  on.  Such 
formulae  have  their  uses  as  expressing  rather 
clearly  certain  definite  facts,  and  as  indicat- 
ing in  a  general  way  some  of  the  probable 
or  possible  processes  which  have  been  con- 
cerned in,  or  have  at  least  influenced,  the 
modification  of  plants  and  animals  in  their 
long  course  of  evolution.  But,  after  all,  they 
are  only  generalised  descriptions,  and  give 
us  very  little  real  or  direct  insight  into  the 
nature  of  the  processes  themselves,  and  yet 
it  is  precisely  in  the  latter  that  the  whole 
secrets  of  evolution,  and  all  that  it  implies, 
are  contained.  In  the  particular  example 
we  have  been  considering,  we  want  to  know 


ADAPTATION  129 

how  the  production  of  the  mechanical  tissue 
is  effected,  and  how  the  remarkable  and 
evident  correspondence  between  its  distribu- 
tion within  the  plant  and  the  various  condi- 
tions imposed  by  the  environment  is  brought 
about. 

It  might  seem  to  be  a  simple  affair  to  pro- 
duce, or  at  least  to  promote,  the  development 
of  mechanical  tissue  by  merely  subjecting  a 
part  of  the  plant  to  an  artificial  stress.  But 
even  if  we  could  do  this,  it  would  still  leave 
the  kernel  of  the  matter  untouched.  As  a 
matter  of  fact,  however,  the  attempt  has 
often  been  made,  but  the  results  have  been 
for  the  most  part  entirely  negative.  Voch- 
ting,  for  example,  endeavoured  to  induce  the 
appropriate  formation  of  strengthening  tissue 
by  attaching  weights  to  plants  in  various  ways, 
and  in  a  number  of  different  positions.  In 
no  single  instance  did  he  get  a  clearly  positive 
result.  But  what  cannot  be  done  by  merely 
applying  an  external  force,  can  readily  be 
accomplished  if  the  requisite  nutritive  func- 
tions, and  perhaps  other  internal  processes 
also,  become  involved.  We  know  how  the 
growth  of  muscle  is  stimulated  by  use, 
consequent,  at  least  in  part,  on  the  better 
nutrition  which  an  improved  condition  of 
circulation  ensures.  An  analogous  instance 
is  furnished  by  plants.  Vochting,  experi- 
menting with  certain  kinds  of  cabbages, 
found  that  after  grafting  heavy  tops  on  to 
younger  and  thinner  stems  the  latter  forth- 


130  PLANT  LIFE 

with  began  to  differentiate  mechanical  tissue, 
appropriate  both  in  form  and  position  to  the 
particular  forces  it  became  necessary  to 
counteract. 

In  another  connection,  it  may  be  observed 
that  new  vascular  tissue  can  be  differentiated 
in  the  leaves  of  some  plants  if  their  vascular 
bundles  are  injured,  and  this  new  tissue  is 
formed  at  the  expense  of  cells  which  hitherto 
have  discharged  other  and  very  different 
functions.  The  union  of  appropriate  tissues 
between  stock  and  scion  in  grafting  furnishes 
yet  another  example.  These  instances  have 
been  mentioned  here  to  avoid  giving  too  one- 
sided an  impression  of  the  evidence  available 
in  connection  with  the  problem. 

Such  experiments  as  those  above  mentioned 
serve  to  throw  a  little  light  on  the  matter, 
by  enabling  us  to  realise  that  the  final  result 
is  due  not  so  much  to  a  process  of  direct 
adaptation  as  to  the  interaction  of  a  number 
oi  different  functions.  These  have  somehow 
or  other  to  be  correlated  within  the  plant,  in 
order  to  produce  the  observed  effect.  Nutri- 
tion obviously  plays  a  part,  though  how  large 
or  important  it  is  we  do  not  know;  but  at 
least  it  is  essential,  if  only  as  providing  the 
means  for  thickening  the  cell  walls.  It  is, 
however,  very  clear  that  the  causes  under- 
lying the  adaptive  character  of  the  distribution 
of  the  tissues  are  still  far  to  seek,  and  much 
more  detailed  analysis  of  the  life  processes 
are  required  before  we  shall  be  able  to  trace, 


PLANTS   AND   WATER  131 

even  in  outline,  the  actual  relations  of  cause 
and  effect.  We  have  as  yet  no  certain  or 
definite  knowledge  of  the  physical  machinery 
of  heredity.  We  do  not  know  why  one  plant 
reacts  in  this,  another  in  that  manner  towards 
an  apparently  identical  set  of  external  con- 
ditions. But  we  have  reasons  for  believing 
that  the  difference  lies  somehow  and  some- 
where in  the  obscurities  of  individual  or  racial 
character  which  in  turn  are  dependent  on 
differences  in  physical  and  chemical  consti- 
tution. But  as  yet  we  can  do  little  more  than 
guess  wherein  the  nature  of  these  differences 
may  lie. 


CHAPTER  XII 

RELATION   OF   PLANTS    TO   WATER 

WE  have  already  become  acquainted  with 
the  manner  in  which  the  ordinary  land  plants 
absorb  the  water  they  require.  Now  water 
plays  so  significant  a  part  in  connection  with 
all  the  principal  functions  of  living  things 


132  PLANT  LIFE 

that  a  closer  examination  of  the  matter  will 
reveal  much  that  is  of  intense  interest,  and 
of  great  importance  in  its  theoretical  bearings 
on  the  problems  already  adumbrated. 

The  urgent  need  of  water,  common  to  all 
vegetation,  is  especially  great  on  the  part  of 
the  green  plants,  although  the  larger  portion  of 
that  which  is  absorbed  is  not  used  directly  in 
the  synthetic  functions,  but  is  exhaled  through 
the  stomata  with  which  most  leaves  are  so 
plentifully  provided.  Its  value  to  the  plant 
stands  even  before  that  of  light,  for  photo- 
synthesis, like  other  characteristically  vital 
functions,  is  practically  arrested  as  soon  as 
the  supply  of  water  falls  below  a  critical 
amount. 

Some  of  the  higher  and  many  of  the  lower 
green  plants  are  able  to  tolerate  long  periods 
of  drought;  but  they  do  so  by  passing  into  a 
condition  of  suspended  animation,  during 
which  many  of  their  chemical  processes 
are  slowed  down  and  others  are  completely 
arrested.  Thus  a  large  number  of  lichens, 
certain  mosses,  and  various  other  plants,  may 
all  become  so  far  desiccated  during  dry  periods 
that  they  can  be  easily  reduced  to  powder. 
A  shower  of  rain,  however,  serves  to  restore 
them  in  a  few  minutes  to  a  condition  of 
renewed  and  active  vitality. 

Nearly  all  land  plants  are  liable  to  encounter 
periods  during  which  the  supply  of  available 
moisture  runs  short.  The  shortage  may  be 
due  to  seasonal  or  climatic  causes,  or  it 


PLANTS  AND   WATER  133 

may  be  incidental  to  the  particular  kind  of 
habitat  in  which  a  plant  is  growing.  For 
example,  plants  which  live  on  bare  rocks,  or 
on  tree  trunks,  are  evidently  exposed  in  a 
greater  degree  to  intermittence  in  water 
supply  than  those  which  are  rooted  in  the 
soil.  We  find  that  such  lithophytic  and  epi- 
phytic vegetation  is  especially  rich  in  species 
that  exhibit  wonderful  adaptations  to  their 
own  particular  environment,  adaptations 
which  enable  them  successfully  to  cope  with 
the  difficulties  and  disadvantages  that  so 
obviously  face  them. 

A  somewhat  wider  survey  of  the  water 
problem  as  it  affects  vegetation  generally, 
shows  that  it  is  necessary  to  distinguish  clearly 
between  that  kind  of  drought  which  is  merely 
physical,  i.  e.  is  due  to  actual  scarcity  of  water, 
and  another  kind  which  may  be  more  properly 
described  as  physiological.  In  the  latter  case 
a  plant,  however  favourably  it  may  seem  to 
be  situated  so  far  as  access  to  water  is  con- 
cerned, may  nevertheless  be  unable  to  absorb 
it  in  sufficient  quantity. 

This  may  happen  when  the  temperature 
of  the  medium  is  too  low;  for  the  active 
absorption  by  roots  is  only  possible  within 
a  rather  narrow  range  of  temperature,  the 
limits  varying  somewhat  for  different  plants. 
Or  the  water  itself  may  contain  substances 
in  solution  which  prejudicially  affect  the 
exercise  of  the  absorptive  functions.  Thus 
the  water  of  salt  marshes,  as  well  as  that 


134  PLANT  LIFE 

which  saturates  the  soil  of  peaty  moorlands, 
is  not  available  for  the  vast  majority  of 
plants,  and  they  are  consequently  precluded 
from  occupying  regions  where  such  conditions 
prevail.  Those  plants  which  do  tolerate  or 
even  demand  them,  often  take  in  relatively 
small  quantities  of  the  water,  and  they  have 
in  consequence  to  limit  the  amount  lost  as 
vapour  in  transpiration.  In  connection  with 
this  limitation  a  variety  of  subsidiary  modifi- 
cations of  habit  may  become  manifest.  Slow- 
ness of  growth,  succulence,  or  the  opposite 
character  of  spininess,  are  common  features; 
whilst  an  evergreen  habit  with  leathery  leaves 
is  of  fairly  frequent  occurrence  amongst  the 
perennial  plants  of  such  localities.  Indeed, 
experience  shows  that  any  circumstance  tend- 
ing to  reduce  the  amount  of  available  water, 
whether  due  to  physical  or  physiological 
conditions,  will  stamp  its  impress  on  the 
vegetation. 

The  onset  of  a  period  of  drought  will 
speedily  result  in  the  extinction  of  entire 
species  within  the  affected  area,  and  their 
places  will  rapidly  be  taken  by  others  which 
are  already  adapted  to  these  new  conditions. 
Which  of  the  many  possible  forms  of  adapted- 
ness  to  drought  a  particular  colonist  may 
possess,  depends  of  course  on  its  own  inherent 
and  hereditary  properties.  The  part  played 
by  the  environment  in  the  matter  is  merely 
to  rule  out  all  those  plants  which  are  not 
previously  fitted  in  one  way  or  another  to 


PLANTS   AND   WATER  135 

conform  to  its  requirements,  and  to  tolerate 
the  limitation  which  it  imposes. 

It  is  necessary,  however,  to  observe  the 
greatest  caution  in  concluding,  as  is  sometimes 
done,  that  the  various  "  adaptations "  to 
dry  conditions  are  to  be  attributed  offhand 
to  a  faculty  assumed  to  be  possessed  by  the 
plant  which  enables  it  to  make  a  direct  and 
appropriate  response  to  the  demands  of  the 
environment  in  question.  As  a  matter  of 
fact  many  plants  Are  incapable  of  making 
any  purposive  response  at  all ;  and  the  matter 
is  by  no  means  a  simple  one  even  in  the  case 
of  those  which  can  so  react.  The  constitution 
of  the  living  protoplasm  is  the  main  factor 
which  determines  the  nature  of  response  to 
water  requirements,  no  less  than  to  mechanical 
needs.  It  is  only  those  plants,  the  living 
substance  of  which  has  become  definitely 
altered  in  certain  (but  alternative)  ways,  that 
are  capable  of  exhibiting  adaptations  (or 
adaptedness)  towards  a  particular  set  of 
external  conditions.  The  successful  reaction 
is  commonly  bound  up  with  complex  internal 
functional  relations,  and  among  these  nutrition 
often  plays  a  leading  share.  It  may  happen, 
as  in  the  formation  of  winter  bud  scales  (see 
p.  142),  that  the  functional  conditions  which 
are  more  immediately  concerned  in  the  forma- 
tion of  "  adaptive  "  structures  ensure  their 
production  quite  independently  of  their  ulti- 
mate utility  as  protective  organs  during  the 
winter  months. 


136  PLANT  LIFE 

The  behaviour  of  plants  at  the  different 
seasons  of  the  year  is  instructive  from  this 
point  of  view. 

The  habit  of  shedding  the  leaves  on  the 
approach  of  winter  which  is  so  characteristic 
of  the  majority  of  our  trees  and  shrubs  is 
often  regarded  as  an  adaptation  to  physiologi- 
cal drought  rather  than  as  directly  due  to 
the  action  of  the  lowering  of  temperature  on 
leaves.  Although  there  is  plenty  of  water 
in  the  soil  in  winter,  the  temperature  of  the 
ground  is  too  low  to  enable  the  trees  to  absorb 
it  freely  enough.  It  is  true  there  are  evergreen 
trees  which  do  not  throw  off  their  leaves  in 
autumn,  but  they  generally  exhibit  definite 
structural  features  indicative  of  a  normally 
slow  rate  of  transpiration,  i.  e.  of  water  lost 
as  vapour  through  the  stomata.  The  leaves 
are  leathery  or  small,  the  stomata  are  compara- 
tively few,  whilst  various  other  features  point 
to  an  economy  in  the  matter  of  water  expendi- 
ture. The  deciduous  trees  and  shrubs,  which 
shed  their  leaves  in  winter,  are  relatively  more 
prodigal  of  water  during  the  warmer  season, 
thus  compensating  for  the  alternate  periods 
of  inactivity. 

Without  doubt  there  is  much  to  be  urged  in 
favour  of  the  deciduous  habit  being  regarded 
primarily  as  an  adaptation  to  a  reduction  of 
the  water  supply.  This  argument  is  strength- 
ened by  a  consideration  of  plants  which 
quite  definitely  respond  to  periodic  drought, 
physical  or  physiological,  by  casting  off  their 


PLANTS   AND  WATER  137 

leaves.  Thus,  in  the  tropics  the  dry  season 
is  marked  by  the  leafless  character  of  many 
trees  which  renew  their  foliage  as  soon  as  the 
rainy  season  sets  in.  Again,  many  evergreens, 
when  transplanted,  frequently  throw  off  their 
leaves.  This  is  the  result  of  injury  to,  and 
disturbance  of,  the  root  system,  whereby 
absorption  is  suddenly  checked.  Hollies  and 
laurels  often  display  this  reaction,  and  indeed 
it  is  generally  to  be  regarded  as  a  favourable 
sign  for  the  future  of  the  plant;  individuals 
that  shed  their  leaves  promptly  always  suffer 
less  than  those  which  retain  their  foliage  in 
a  flaccid  or  withered  condition  on  the  branches. 
But  if  we  look  a  little  further  into  this 
question  of  leaf  fall,  it  turns  out  to  be  not  so 
simple  as  it  appears  at  first  sight.  It  must  be 
premised  that  the  fall  of  the  leaf  is  not  a 
matter  of  mere  detachment,  but  it  ensues  in 
consequence  of  definite  changes  which  have 
caused  a  layer  of  tissue  to  become  differen- 
tiated across  the  base  (usually)  of  the  leaf. 
Thus,  even  before  the  detachment  of  the  leaf, 
the  wound  is  practically  healed  in  advance. 
Although  various  functions  connected  with 
nutrition  are  concerned  in  bringing  about  the 
formation  of  this  "  separation  layer,"  the  most 
powerful  stimulus  is  unquestionably  that  of 
physiological  water  starvation,  whether  this 
starvation  results  from  physical  shortage  or 
from  a  physiological  inability  to  absorb.  The 
intermittent  periods  of  drought  in  summer 
are  often  followed  by  early  leaf  fall  on  the 


138  PLANT  LIFE 

part  of  the  more  intolerant  trees  such  as  the 
lime.  The  evergreens,  on  the  other  hand,  are 
usually  very  long  suffering,  but,  as  we  have 
seen,  a  severe  diminution  of  water  supply  is, 
or  may  be,  followed  by  the  hurrying  up  of 
those  internal  processes  which  culminate  in 
the  differentiation  of  the  separation  layer  at 
the  base  of  the  leaves. 

Thus  a  plant  which  is  fitted  for  average 
conditions  of  water  supply  (and  is  often  there- 
fore called  a  Mesophyte)  may  assume  certain 
of  the  distinctive  characters  of  plants  fitted 
for  dry  conditions,  when  its  supplies  of  water 
are  from  any  cause  suddenly  interfered  with. 
Plants  which  are  specially  adapted  to  dry 
conditions  are  called  Xerophytes,  and  they 
are  directly  contrasted  with  the  Hygrophytes, 
i.  e.  with  those  restricted  to  very  wet 
surroundings. 

In  the  examples  we  have  just  considered, 
the  adaptedness  to  dry  or  xerophytic  condi- 
tions is  attained  by  reduction  of  the  transpiring 
surface.  This  is  a  very  common  feature  of 
xerophytes,1  and  it  forcibly  illustrates  the 
limitation  of  one  important  function  (that  of 

1  Not  all  plants  with  reduced  leaf  surface  are  xero- 
phytes. The  large  Water  Rush  (Scirpus  lacustris)  used  in 
the  manufacture  of  rush-bottomed,  chairs  is  an  instance. 
The  reduction  of  the  leaves,  and  the  transference  of  the 
photosynthetic  function  to  the  stems  of  these  plants  is 
certainly  to  be  correlated  with  mechanical  requirements. 
A  plant  built  on  the  plan  of  the  water  rush  would 
be  an  impossibility  if  any  weight  of  green  foliage  had  to 
be  sustained. 


PLANTS  AND  WATER  139 

photosynthesis)  by  another  factor  which  also 
influences  the  process  of  nutrition  as  a  whole. 

The  reduction  of  leaf  surface,  which  is  of 
intermittent  or  annual  occurrence  in  our  every- 
day vegetation,  may  become  the  normal  state 
of  a  more  specialised  xerophyte.  The  leaves 
may  be  very  small,  or  they  may  even  be 
practically  absent  so  far  as  photosynthetic 
function  is  concerned.  In  such  plants,  how- 
ever, this  office  may  continue  to  be  discharged 
by  the  stems,  which  remain  green,  and  thus 
to  some  extent  may  take  the  place  of  the 
leaves.  Their  special  advantage  in  this  con- 
nection is  mainly  due  to  their  structure,  and 
to  the  relatively  small  number  of  stomata, 
which  enables  them  to  check  the  escape  of 
water  from  the  plant. 

It  is  a  singular  fact  that  when  a  species 
or  race  has  once  exhibited  a  tendency  towards 
the  loss  or  atrophy  of  an  organ,  e.  g.  the  leaf, 
the  descendants  commonly  appear  to  be  un- 
able to  check  it.  If  any  of  them  vary  in  such 
a  way  as  to  increase  their  green  surface,  this 
is  effected  not  by  enlarging  their  diminished 
leaves,  but  by  flattening  and  specialising 
some  other  organ.  Sometimes  the  process 
may  even  be  seen  to  accompany  the  diminu- 
tion of  the  leaves,  as  in  some  species  of  acacia. 
In  Acacia  melanoccylon,  for  example,  we  find 
the  leaf  stalk  gradually  flattening,  and 
assuming  the  functions  generally  undertaken 
by  the  blade,  which  becomes  completely 
atrophied.  Other  species  of  acacia  show  the 


140  PLANT  LIFE 

same  tendency  in  a  still  more  advanced  degree. 
Only  a  few  of  the  earliest  leaves  on  the  seed- 
ling exhibit  a  blade,  all  the  succeeding  ones 
having  flattened  petioles  only. 

More  often,  however,  it  is  the  stem  which 
undergoes  modification  and  develops  leaf- 
like  characters.  When  only  certain  branches 
become  specialised  in  this  way,  as  in  species 
of  Butcher's  Broom  (Ruscus),  it  may  require 
careful  examination  to  detect  the  cauline 
nature  of  the  apparent  leaves.  But  the 
genuine  leaves  are  really  present,  and  although 
they  are  reduced  to  small  brown  scales,  they 
suffice  to  indicate  the  true  condition  in  this 
as  in  other  extreme  examples. 

A  still  more  remarkable  modification  is 
seen  when  the  roots  assume  the  functions 
of  green  leaves.  An  instance  of  this  is  fur- 
nished by  the  genus  of  epiphytic  orchids  known 
as  Taeniophyllum  (Fig.  18).  These  orchids, 
which  possess  very  inconspicuous  flowers,  are 
also  destitute  of  foliage  leaves.  But  the 
function  of  photosynthesis  is  discharged  by 
the  green,  band-  or  tape-like  roots  which  are 
appressed  to  the  bark  of  the  trees  upon  which 
the  plants  are  growing.  In  some  species 
the  roots  are  very  long,  and  hang  freely  from 
the  tree  trunk,  when  their  resemblance  to 
narrow  strap-shaped  leaves  becomes  addition- 
ally striking. 

It  often  happens  that  new  structural  modifi- 
cations— adaptations  in  the  making,  as  it  were 
— respond  to  the  influence  of  the  stimulus 


PLANTS  AND   WATER 


141 


of  light   in  a  remarkably  purposive  fashion. 
Thus  nearly  all  the  cacti,  in  their  adult  stages, 


Fig.  18. — Taeniophyttum  torricellense,  an  orchid  with  green, 
flattened,  leaf -like  roots.  R,  roots;  V,  vascular  strand; 
F,  flowering  spike.  (The  roots  are  shown  broken  off  at 
the  ends;  they  are  actually  much  longer  than  indicated 
in  the  diagram,  which  was  made  from  a  specimen  in  the 
Natural  History  Museum,  S.  Kensington.) 

are  destitute  of  green  leaves,  their  stems 
functioning  as  the  photosynthetic  organs. 
Some  of  them,  like  the  species  of  Phyllo- 


142  PLANT  LIFE 

cactus,  often  grown  for  the  sake  of  their 
beautiful  flowers,  bear  a  strong  resemblance 
to  lobed,  fleshy  leaves.  They  are,  however, 
merely  stems,  flattened  by  the  action  of 
light,  which  is  probably  indirectly  operative 
through  nutritive  processes.  If  the  phyllo- 
cacti  are  grown  in  the  dark  they  only  produce 
rod-like  stems,  very  different  from  the  leaf- 
like  shape  assumed  under  ordinary  conditions 
of  illumination. 

The  cacti  furnish  a  remarkable  range  of 
forms.  All  of  them  are  pronounced  xero- 
phytes.  Now  it  is  a  curious  fact  that  these 
plants  of  the  Western  hemisphere  have  their 
doubles  in  some  of  the  euphorbias,  or  spurges, 
of  the  hot,  dry  regions  of  the  Eastern  tropics. 
So  faithfully,  indeed,  are  many  of  the  leading 
types  of  cactus  forms  reproduced,  that  an 
untrained  observer  may  easily  be  deceived. 
No  satisfactory  explanation  has  been  given  for 
the  occurrence  of  these  closely  similar  forms 
of  plants,  which  are  widely  sundered  in 
affinity  and  properties,  as  well  as  in  their 
geographical  distribution. 

The  ordinary  winter  buds  of  our  trees  and 
shrubs,  with  their  protective  scales,  provide 
us  with  still  other  adaptations  by  which  loss 
of  water  from  the  enclosed  leaves  is  prevented. 
The  young  leaves,  since  they  are  not  only  very 
thin,  but  are  imperfectly  protected  by  a  cuti- 
cular  surface,  would  assuredly  suffer  if  they 
were  exposed  to  the  air.  This  is  probably 
the  chief  function  of  the  bud  scales,  though 


PLANTS  AND  WATER  143 

doubtless  they  further  serve  to  protect  the 
delicate  leaves  within  from  a  variety  of  other 
injurious  influences.  Nevertheless,  in  spite 
of  their  wonderfully  perfect  adaptation  to 
these  functions,  they  may  be  shown  to  owe 
their  existence  in  their  present  form  to  certain 
conditions  which  affect  nutrition  during  a 
far  earlier  period  of  active  vegetation,  for 
they  are  simply  modified  leaves  or  parts  of 
leaves.  In  a  sense  they  may  be  said  to  have 
been  starved  and  arrested,  or  rather  dis- 
torted, in  their  development.  This  can  easily 
be  proved  in  such  a  plant  as  a  young  ash  or  a 
plum,  by  pulling  off  the  young  and  active 
outer  foliage  leaves.  The  operation  must  be 
performed  quite  early  in  the  summer  while 
the  bud  scales  are  young,  and  before  they  are 
fully  formed.  The  result  of  the  experiment 
is  to  cause  the  young  leaves  that  otherwise 
would  have  been  destined  to  form  the  bud 
scales,  to  grow  out,  and  provide  a  further  crop 
of  foliage  leaves.  The  removal  of  the  active 
leaves  has  diverted  nutrition  into  the  young 
bud  scales  that  were  to  be,  and  has  caused 
them  to  assume  the  form  and  character  of 
foliage  leaves;  and  this  happens  for  precisely 
the  same  material  reasons  that  the  foliage 
leaves  themselves  are  forced  to  assume  their 
own  proper  form. 

In  many  parts  of  the  world  the  climate  is 
sharply  marked  into  a  wet  and  a  dry  season. 
During  the  wet  period,  vegetation  of  a  meso- 
phytic  type  can  exist ;  but  unless  it  has  some 


144  PLANT  LIFE 

special  adaptation  to  tide  it  over  the  dry,  and 
often  hot,  time  of  year  it  would  be  unable 
to  occupy  such  climatal  regions.  Often  the 
adaptation  is  fairly  obvious.  Thus,  when  the 
rain  falls  on  the  South  African  veldt,  in- 
numerable leaves  and  flowers  spring  up  as  if 
by  magic.  They  flower  and  fruit,  and  then 
disappear  for  the  rest  of  the  year.  A  re- 
latively large  proportion  of  this  vegetation 
consists  of  perennial  plants  of  a  bulbous 
or  tuberous  character.  As  long  as  the  dry 
weather  lasts  they  remain  in  a  resting  condi- 
tion, the  bulbs  or  tubers  showing  no  sign  of 
life.  If  they  be  cut  open,  they  are  found,  even 
at  this  season,  to  be  juicy ;  that  is  to  say,  they 
store  water  and  retain  it  with  great  tenacity. 
When  conditions  become  favourable,  the 
leaves  rapidly  develop,  and  they  are  often  not 
at  all  the  leathery  or  even  succulent  structures 
one  might  expect  to  meet  with.  In  fact  they 
frequently  resemble  those  of  typically  meso- 
phytic  vegetation,  and  are  thus  simply 
adapted  for  an  average  water  supply,  and 
indeed  such  conditions  do  actually  prevail 
during  their  period  of  active  growth.  Their 
food  manufacture  goes  on  rapidly,  and  the 
surplus  is  stored  up  in  the  swollen  portion, 
so  that  when  the  growing,  moist  season  is 
over,  they  will  have  accumulated  an  amount 
of  easily  utilisable  food.  It  is  the  possession 
of  these  qualities  which  enables  them  to 
form  underground  the  flowers  and  leaves 
which  will  expand  so  rapidly  on  the  return 


PLANTS   AND   WATER  145 

of  the  rains.  Everything  is  almost  complete, 
and  is  ready  to  push  above  the  ground  at  a 
few  days',  or  even  a  few  hours',  notice. 

This  form  of  response  to  periods  of  drought, 
namely  the  capacity  to  store  up  food,  and 
even  water,  is  very  widespread,  but  one  must 
not  imagine  that  all  bulbous  plants  are  to  be 
looked  on  as  xerophytes,  though  the  bulbous 
habit  undoubtedly  does  confer  on  its  possessor 
the  power  or  faculty  of  colonising  localities 
such  as  those  just  indicated.  Many  of  our 
spring  woodland  plants  are  bulbous  or 
tuberous ;  but  in  their  case  it  is  not  so  much 
a  question  of  drought  as  one  of  light. 

The  bulbous  character  of  the  wild  hyacinth, 
for  example,  enables  it  to  thrive  in  shady 
woods,  even  under  beech  and  hornbeam,  for, 
like  its  relatives  in  the  open  field,  it  is  provided 
with  a  large  stock  of  available  food  in  the 
bulb  scales,  which  was  manufactured  and 
stored  up  during  the  preceding  spring. 
When  the  warm  weather  returns  after  the 
winter  the  hyacinths  rapidly  sprout,  and  their 
green  leaves  are  fully  exposed  to  the  light. 
Later  on,  however,  as  the  trees  unfold  their 
leaves  the  light  soon  weakens,  and  little  or 
no  photosynthesis  can  go  on  under  the  dense 
shade  of  a  beech  wood.  But  by  this  time  the 
plants  have  done  their  work,  and  have  already 
laid  up  a  stock  of  food  for  the  following  year. 
Their  leaves  die  down,  and  only  the  ripening 
seed  capsules  reveal  their  presence  in  the 
wood. 


146  PLANT  LIFE 

The  hyacinth  is  by  no  means  necessarily 
a  woodland  plant.  In  many  parts  of  Wales 
and  Scotland  it  grows  amongst  the  grass  in 
the  open  fields,  wherever  it  is  able  to  compete 
with  the  growing  herbage. 

The  point  which  the  hyacinth  enables  us 
to  emphasise  is  this,  that  whilst  the  bulbous 
(or  tuberous)  habit  is  one  which  will  put  its 
owner  into  favourable  relation  with  certain 
types  of  dry  climates,  it  will  also,  and  for 
analogous  reasons,  prove  an  adaptation 
suitable  for  other  and  very  different  conditions 
as  well,  provided  that  these  also  include  a 
brief  period  favourable  to  the  vegetative 
activity  of  the  plants. 

When  the  climate  is  persistently  dry  the 
vegetation  is  usually  mixed  and  consists  of 
plants  which  are  either  succulent  or  spiny. 
These  seem  contradictory  features,  and  in  a 
measure  so  they  are.  Nevertheless,  each 
habit,  that  of  succulence  and  that  of  spininess, 
is  well  adapted  for  dry  climatic  conditions. 
The  succulent  plant  stores  what  water  it  can 
get  and  when  it  can  get  it.  A  remarkably 
extensive  and  deep  root  system  is  often 
developed,  by  which  it  is  enabled  to  search 
the  ground  thoroughly  and  effectively  for  the 
requisite  moisture.  Moreover,  such  water  as 
it  does  acquire  is  lost  very  slowly,  owing  to 
peculiar  features  connected  with  the  stomata. 
The  presence  of  wax  or  "  bloom  "  also  serves 
as  an  additional  check  to  the  escape  of  watery 
vapour. 


PLANTS  AND   WATER  147 

The  spiny  plants  are  similarly  built  on  lines 
calculated  to  limit  the  output  of  water, 
though  why  the  reduced  branches  and  leaves 
should  so  commonly  assume  the  form  of 
spines  it  is  not  easy  to  say.  The  supposed 
function  of  the  spines  in  keeping  off  browsing 
animals  is  a  ready,  but  not  very  satisfying, 
explanation. 

Observation  teaches  that  both  classes  of 
plants,  the  spiny  and  succulent,  are  of  com- 
paratively slow  growth.  But  it  is  not  quite 
correct  to  assert  of  xerophytes  generally  that 
in  their  habits  and  rate  of  growth  they  com- 
pare unfavourably  with  the  mesophytes.  The 
truth  rather  is  that  they  are  able  if  need  be 
to  support  life  on  a  very  limited  income,  by 
cutting  down  their  expenditure  in  various 
directions.  It  is  by  this  faculty  of  exercising 
economy  that  they  are  enabled  to  flourish  in 
regions  from  which  the  less  hardy  mesophytes 
are  excluded.  A  large  number  of  xerophytes 
are,  however,  by  no  means  solely  adapted  to 
a  life  of  austerity.  Transplanted  and  grown 
under  ameliorated  conditions,  they  often 
respond  to  the  change  by  a  rapid  and  vigorous 
growth.  It  seldom  happens,  however,  that 
they  are  able  to  hold  their  own  in  competi- 
tion with  the  mesophytes  in  a  natural  en- 
vironment suitable  for  the  latter,  and  they 
commonly  become  killed  out  sooner  or  later 
by  their  more  vigorous  rivals. 

There  are  a  few  of  the  highly  specialised 
xerophytes,  such  as  cacti,  which  are  so 


148  PLANT  LIFE 

definitely  modified  in  relation  to  conditions 
of  drought  that  they  have  become  extremely 
intolerant  of  moisture,  even  in  quantities  such 
as  would  barely  suffice  to  keep  an  ordinary 
mesophyte  alive.  Plants  such  as  these 
stand  at  one  extreme  end  of  the  scale  of 
vegetation,  the  other  end  being  occupied  by 
the  genuine  aquatics  or  hydrophytes  which 
also  are  unable  to  endure  mesophytic  con- 
ditions, because  they  lose  water  too  readily, 
Different  as  are  these  extreme  examples  from 
one  another,  they  yet  agree  in  this  respect, 
namely  that  the  chemical  processes  character- 
istic of  their  vital  functions  are  incapable  of 
becoming  so  modified  as  to  produce  the  kind 
of  structure  suited  to  average  mesophytic 
conditions.  In  the  case  of  aquatics  the 
general  nature  of  this  defect  is  clearer  than 
in  the  xerophytes,  and  mainly  depends  on 
the  inability  to  form  a  suitable  cuticle,  added 
to  which  the  functions  of  water  conduction 
and  mechanical  support  are  often  inadequate 
for  a  terrestrial  habitat. 


THE  EPIPHYTES  149 


CHAPTER  XIII 

THE    EPIPHYTES 

HITHERTO  we  have  chiefly  considered  the 
relation  of  vegetation  to  an  exiguous  water 
supply  rather  from  the  point  of  view  of  par- 
simony. A  short  or  precarious  supply  is 
met  by  reducing  the  output,  hoarding  the 
precious  liquid,  or  living  an  abstemious  life. 

But  other  plants  have  shown  greater  powers 
of  invention,  so  to  speak,  in  overcoming 
the  difficulties  of  life.  They  have  countered 
intermittence  by  the  construction  of  more  or 
less  ample  cisterns,  and  they  have  developed 
new  methods  by  which  the  available  water  is 
absorbed.  These  modifications  of  structure 
have  rendered  existence  possible,  and  even 
easy,  in  many  situations  from  which  ordinary 
plants  are  debarred  from  establishing  them- 
selves. Perhaps  the  best  examples  of  this 
inventive  resourcefulness  are  to  be  met  with 
amongst  the  plants  that  have  exchanged  a 
terrestrial  for  an  arboreal  habitat.  Such, 
plants  are  generally  called  epiphytes.  They 
are  in  no  sense  necessarily  parasitic,  that  is 
to  say  they  do  not  tax  their  host  for  food. 
All  they  demand  from  the  trees  is  the  space 
whereon  to  grow. 


150  PLANT  LIFE 

The  epiphytes  form  a  large  class,  and  they 
include  many  of  the  humbler  members  of 
the  vegetable  kingdom  as  well  as  a  consider- 
able number  drawn  from  the  highest  ranks  of 
flowering  plants. 

They  exhibit  all  grades  of  adaptedness  for 
the  acquisition  and  storage  of  water.  At  the 
lowest  end  we  find  some  of  the  simpler  forms, 
especially  amongst  the  algae  and  mosses, 
which  will  stand  complete  dessication.  But 
there  are  other  species  of  mosses,  and  especially 
of  the  nearly  related  family  of  liverworts, 
which  have  advanced  far  beyond  the  attitude  of 
mere  tolerance,  and  not  only  exhibit  adapta- 
tion for  rapid  water  absorption,  but  also 
possess  means  of  storing  it  during  a  time  of 
plenty.  In  some  liverworts  tuberous  bodies 
are  formed,  and  during  the  dry  season  these 
alone  persist,  to  break  out  into  growth  as 
soon  as  the  rains  commence.  In  the  leafy 
forms  it  often  happens  that  some  or  all  of 
the  leaves  are  modified  so  as  to  form  bottle- 
like  receptacles  (Frullania,  Physiotium,  etc.) 
for  water. 

It  is  amongst  the  ferns  and  flowering  plants, 
however,  that  we  find  the  greatest  diversity, 
and  perhaps  we  might  add  perfection,  in  the 
adaptations  to  solve  the  problems  connected 
with  a  precarious  and  intermittent  water 
supply. 

It  is  true  that  the  majority  of  the  highly 
specialised  epiphytes  are  more  or  less  restricted 
to  regions  of  large  and  fairly  frequent  rainfall, 


THE  EPIPHYTES  151 

but  others  are  able,  owing  to  certain  peculi- 
arities of  structure  and  habit,  to  endure 
recurrent  periods  of  drought  provided  that 
they  do  not  suffer  too  much  in  this  respect 
during  their  season  of  active  vegetative  growth. 

Disregarding,  then,  the  less  highly  specialised 
epiphytes,  which  respond  to  a  dry  season 
by  simply  closing  down  their  vital  processes, 
we  will  turn  our  attention  to  the  more  highly 
adapted  types  amongst  the  flowering  plants. 

The  orchids  will  serve  as  our  first  examples. 
A  large  proportion  of  the  members  of  this 
family  are  not  epiphytic  at  all,  but  grow  in 
the  ground.  Even  there  they  exhibit  many 
deviations  from  the  typical  structure  of  roots, 
but  in  a  number  of  the  epiphytic  species,  so 
common  in  the  tropical  forests,  the  root- 
system  undergoes  a  remarkable  and  adaptive 
change  of  structure.  Whilst  some  of  the 
roots  may  depart  but  slightly  from  the  form 
commonly  met  with  in  these  organs,  and 
serve  to  fasten  the  plant  to  its  arboreal  perch, 
others  are  thicker,  often  green,  and  when 
dry  are  of  a  white  or  lustrous  grey  colour 
The  whiteness  is  due  to  the  presence  of  air 
in  the  outer  layers  of  cells  which  form  a  very 
peculiar  sheathing  mantle  on  the  root.  In 
ordinary  roots  there  is  but  one  well-defined 
layer  sheathing  the  rind  and  giving  rise  to  the 
root- hairs;  but  in  these  orchids  it  divides 
and  forms  many  layers,  whilst  the  root- hairs 
are  usually  suppressed.  The  illustration  will 
better  explain  what  is  meant,  and  will  serve 


152  PLANT  LIFE 

to  bring  out  the  more  salient  features  of  this 
remarkable  structure,  which  is  generally  known 
as  the  velamen  of  the  orchid  root  (Fig.  19). 

The  cell  walls  of  the  velamen  are  strength- 
ened by  bars  of  thickening,  which  gives  them 
a  spiral  or  netted  appearance  under  the 
microscope.  The  function  of  the  velamen 
as  a  whole  is  to  act  as  a  sort  of  sponge  which 
soaks  up  liquid  falling  on  it  with  extreme 
rapidity.  Thus  the  plant  is  able  quickly  to 
replenish  its  supplies  of  water  during  a  shower. 
In  many  orchids  the  bases  of  the  stems  and 
sometimes  the  leafy  joints  are  swollen  with 
the  so-called  "  pseudobulbs,"  which  form 
additional  storehouses  for  the  water  thus 
obtained.  During  the  periods  which  inter- 
vene between  the  rains,  the  plant  often 
throws  off  its  leaves,  its  flowers  drawing  on 
the  water  supplies  stored  in  the  pseudobulb. 
The  surface  of  the  latter  becomes  more  and 
more  wrinkled  as  its  storage  cells  become 
depleted  of  their  water  contents. 

The  roots,  although  specialised  in  the  way 
described  above,  have  retained,  in  many  in- 
stances at  any  rate,  the  power  of  growing  like 
ordinary  ones  if  they  should  happen  to  pene- 
trate the  substratum.  This  sometimes  occurs 
when  there  is  sufficient  vegetable  detritus 
caught  in  the  orchid  clump,  or  when  the  root 
penetrates  a  piece  of  damp  rotten  wood. 
Root- hairs  are  then  produced,  and  the  velamen 
may  be  scarcely  produced  at  all. 

It  is  a  remarkable  fact  that  some  of  the 


Fig.  19. — Transverse  section  of  an  orchid  (Dendrobium)  root 
showing  the  velamen  (V).  Note  the  thin-walled  "  passage- 
cells  "  (P),  through  which  the  water  gains  access  to  the 
interior  of  the  root. 

163 


154  PLANT  LIFE 

epiphytic  members  of  a  very  different  family, 
the  aroids,  have  also,  and  independently, 
acquired  the  faculty  of  developing  a  velamen 
which  is  closely  similar  to  that  formed  by 
the  orchids. 

It  is  very  easy  to  see  that  the  presence  of 
velamen  is  of  great  use  to  a  plant  growing 
as  an  epiphyte,  but  that  is  not  at  all  the  same 
thing  as  accounting  for  its  presence.  It 
certainly  enables  the  plant  to  take  advantage 
of  such  positions  as  the  trunks  of  trees,  where 
it  becomes  lifted  up  to  the  light,  and  enjoys 
various  other  advantages.  But  how  has  it 
come  about  that  it  is  just  developed  in  these 
orchids  (and  aroids)  in  response  to  their 
particular  needs,  whilst  the  innumerable 
epiphytes  belonging  to  other  families  of 
plants  have  not  altered  their  roots  in  this 
striking  manner  ?  We  cannot  tell — at  any 
rate  at  present. 

The  same  difficulty  in  giving  a  real  explana- 
tion is  inherent  in  every  problem  of  plant  (and 
animal)  form,  but  it  is  often  slurred  over, 
especially  when  the  structure  is  obviously  of 
use  in  a  particular  connection.  To  describe 
it  as  an  adaptation  to  a  particular  condition 
of  the  environment  is  merely  to  state  an 
impression.  Such  descriptive  phrases  furnish 
no  explanation  of  origin,  nor  do  they  illum- 
inate in  any  material  degree  the  hidden 
relations  of  cause  and  effect. 

There  are  other  flowering  epiphytes  which 
absorb  water  not  by  their  roots  at  all,  but 


THE  EPIPHYTES  155 

by  curiously  developed  hairs  on  their  leaves. 
Examples  of  this  habit  are  afforded  by  the 
Tillandsias,  and  other  Bromeliads  of  the 
tropical  forests  of  the  Western  world.  The 
roots  may  be  altogether  lacking  in  some  species, 
and  even  when  they  are  produced  they  merely 
serve  to  attach  the  plant  to  a  branch,  and 
function  only  slightly,  or  not  at  all,  as  water- 
absorbing  organs.  Tillandsia  usneoides,  com- 
mon in  the  damp  West  Indian  forests,  pos- 
sesses no  roots ;  it  bears  sickle-shaped  leaves 
which  readily  become  entangled  in  small 
twigs,  and  the  greyish-green  festoons  of  this 
plant,  as  they  hang  down  from  the  branches, 
resemble  luxuriant  lichens  rather  than  a 
flowering  plant.  Indeed,  the  resemblance  is 
so  great  as  readily  to  deceive  any  but  a  care- 
ful observer. 

The  epiphytic  tillandsias  absorb  the  whole 
of  their  water  supply  through  remarkable  hairs 
which  clothe  the  surfaces  of  the  plant.  The 
accompanying  figure  illustrates  their  general 
appearance  and  structure  (Fig.  20).  From  a 
slight  depression  there  arises  a  stalked  hair, 
the  upper  portion  of  which  is  flattened  out 
as  a  membranous  expansion  consisting  of 
many  cells  arranged  around  the  central  group 
that  terminates  the  stalk.  The  cell  walls 
on  the  upper  surface  of  the  hair  are  very 
thick,  but  they  are  practically  destitute  of 
a  cuticle,  and  water  probably  can  pass 
through  them  as  well  as  through  the  walls 
on  the  under  surface  which  are  much  thinner. 


156 


PLANT  LIFE 


The  flattened  membranous  tops  of  the  hairs 
often  overlap,  and  when  water  is  dashed  on 


E 

Fig.  20. — Tillandsia  usneoides.  I. — A  portion  of  the  leafy 
stem,  showing  the  curved  form  of  the  stem  and  leaves. 
II. — One  of  the  hairs  on  the  leaf  in  section  (highly  magni- 
fied). The  water  which  is  soaked  up  by  the  hair  passes 
by  the  darkly  shaded  cells  into  the  interior  of  the  leaf. 
E,  epidermis  of  the  leaf. 

the  plant  it  is  held  amongst  them  by  capillary 
attraction,  the    displacement    of    the  air  by 


THE  EPIPHYTES  157 

water  causing  the  grey  colour  to  turn  green. 
Water  rapidly  passes  into  the  cells  of  the  hair 
and  thence  is  transmitted  to  the  leaf.  As 
the  air  becomes  dry  again  the  hair  gradually 
flattens  down,  and  its  thick  outer  wall  serves 
as  an  additional  barrier  to  prevent  undue 
loss  of  water  from  the  leaf. 

It  is  obvious  that  tillandsias,  and  other 
plants  which,  like  them,  grow  on  the  branches 
of  trees,  must  be  largely  dependent  on  some- 
what casual  sources  for  the  small  supply  of 
mineral  salts  which  are  required  for  their 
subsistence.  Most  of  it  reaches  them  in  the 
form  of  dust,  or  as  vegetable  detritus  of 
various  kinds.  Inasmuch  as  the  leaf -hair, 
like  the  root-hair,  can  only  absorb  substances 
already  in  solution,  it  becomes  a  question 
of  some  interest  to  ascertain  whether  the 
salts  really  do  pass  into  the  plant  in  this 
way,  and  if  so  whether  the  absorbent  hairs 
of  the  epiphytic  tillandsias  differ  in  this 
respect  from  their  near  relatives  which  still 
grow  in  the  ground. 

It  has  been  ascertained  that  salts  are  so 
taken  in  by  the  epiphytic  species,  but,  as 
might  be  expected,  not  necessarily  or  com- 
monly so  by  the  rest.  The  Pine-apple,  a 
terrestrial  plant  related  to  Tillandsia,  possesses 
hairs  similar  to  those  of  the  latter  plants,  on 
its  leaves.  They  are  able  to  absorb  water, 
but  the  salts  dissolved  in  it  do  not  pass  in, 
for  they  cannot  traverse  the  protoplasmic 
lining  of  the  cells. 


158  PLANT  LIFE 

Many  of  the  other  members  of  the  natural 
order  (Bromeliacese)  to  which  the  pine-apple 
and  the  tillandsias  belong,  are  variously 
specialised  with  respect  to  water  supplies. 
The  large  bromeliad  of  tropical  America 
possess  clasping  roots  which  fasten  the  short 
stem  with  its,  relatively  speaking,  immense 
crown  of  foliage,  on  the  branch  of  the  tree 
which  serves  as  its  perch.  The  roots  have  little 
if  anything  to  do  with  absorption,  a  function 
which  has  been  almost  entirely  taken  over 
by  the  tillandsia-like  hairs  which  are  found 
on  the  upper  surfaces  of  the  leaves.  The 
latter  form  remarkable  cisterns  in  which 
water  is  collected  and  stored,  and  from  which 
it  is  absorbed  by  the  hairs  that  are  especially 
numerous  where  the  water  is  stored.  Each 
leaf  is  a  long,  more  or  less  strap-shaped  body, 
with  the  edges  curving  towards  each  other  in 
the  middle  portion,  thus  forming  a  sort  of 
gutter.  Nearer  the  base,  the  leaves  press 
tightly  on  each  other  and  thus  constitute 
the  cisterns.  Water  falling  on  the  upper 
surface  of  the  leaf  is  directed  into  them  by 
means  of  the  gutter-like  curvature  just 
described,  which  causes  the  rain  to  run  down 
to  the  centre  of  the  crown,  instead  of  dripping 
off  as  it  does  from  most  leaves.  The  efficient 
manner  in  which  the  foliage  is  arranged  to 
form  cisterns  may  be  gauged  by  the  fact  that 
water  plants  are  fairly  often  found  growing 
and  flourishing  in  the  water  thus  collected 
and  retained. 


THE  EPIPHYTES  159 

Another  plant,  Dischidia  rafflesiana,  be- 
longing to  the  very  different  family  of  Ascle- 
piads,  may  be  considered  in  this  connection 
though  it  is  not  strictly  speaking  an  epiphyte. 
It  commonly  grows  on  rocky  surfaces,  but 
it  is  subjected,  like  the  epiphyte,  to  the  need 
of  special  adaptations  for  obtaining  water. 
Its  ordinary  leaves  are  rather  thick  and  fleshy, 
and  thus  are  to  be  regarded  as  dealing  econo- 
mically with  such  water  as  may  be  available. 
But  some  of  its  leaves  undergo  a  most 
remarkable  modification  in  the  course  of 
their  development,  and  assume  the  form 
of  pitchers.  The  mouth  of  the  pitcher  is 
directed  upwards,  and  they  are  readily 
filled  by  the  heavy  showers  that  prevail  in 
the  regions  of  the  Eastern  Archipelago  and 
Malay  where  the  plant  occurs.  The  utilisa- 
tion of  the  water  is  finally  effected  by  small 
branching  roots,  which  spring  from  the  stem 
close  to  the  insertion  of  the  leaf  pitcher. 
These  enter  it  and  ramify  inside  it.  Often 
detritus  of  various  sorts  becomes  washed  into 
the  pitcher,  and  thus  it  not  only  serves  to 
collect  water,  but  it  actually  provides  soil 
for  the  plant  as  well. 

A  few  of  the  epiphytes  have  been  chosen 
for  consideration  here  because  they  so  admir- 
ably illustrate  the  remarkable  methods  by 
which  the  difficulties  of  obtaining  water  have 
been  overcome.  An  extended  study  of 
these  remarkable  plants  would  have  shown 
this  more  in  detail,  for  there  is  hardly  any 


160  PLANT  LIFE 

conceivable  device  which  has  not  been  actually 
put  into  practice  by  one  or  another  of  them. 
But  whatever  be  the  particular  nature  of 
the  adaption  in  any  given  instance,  its  final 
significance,  and  its  ultimate  justification,  is 
to  be  sought  in  the  assistance  it  renders  towards 
the  proper  discharge  of  the  photosynthetic 
functions  of  the  green  surface. 

Plants  which  contain  no  chlorophyll — and 
there  are  vast  numbers  of  them — have  no 
use  for  the  varied  complications  that  circum- 
stances may  render  essential  or  ancillary  to 
the  green  members  of  the  vegetable  kingdom. 
Indeed,  it  is  not  too  much  to  say  that  all  the 
beauty  of  form,  all  the  elaborate  structure, 
which  is  so  copiously  displayed  by  the  vegeta- 
tive organs  of  trees  and  plants  generally, 
spring  from  the  possession  of  this  substance 
chlorophyll,  with  its  wonderful  power  of 
trapping  and  utilising  the  energy  of  the 
sunlight. 

But  apart  from  all  aesthetic  considerations, 
and  regarding  the  matter  solely  from  the 
most  material  point  of  view,  we  may  further 
assert  that  the  elaboration  of  chlorophyll 
has  been  fraught  with  consequences  to  the 
whole  organic  world  compared  with  which 
all  the  other  structural  products  of  evolu- 
tionary change  sink  into  insignificance  and 
obscurity. 


THE   FUNGI  161 


CHAPTER  XIV 

THE  FUNGI 

WE  may  now  turn  our  attention  to  a  very 
different  type  of  vegetation,  the  members 
of  which  are  wholly,  or  almost  wholly,  lacking 
in  chlorophyll.  The  fungi  and  bacteria  are 
typical  representatives  of  these  non-green 
organisms,  but  a  certain  number  of  the 
higher  flowering  plants  have  also  departed 
from  the  way  of  their  ancestors,  and  have 
lost  their  green  colour.  It  is  a  significant 
fact,  which  strikes  us  at  the  very  outset,  that 
every  one  of  these  colourless  flowering  plants 
live  on  ready-formed  organic  matter  derived 
either  from  dead  or  still  living  organisms. 

As  a  class,  the  plants  which  lack  chlorophyll 
are  distinguished  by  the  relatively  simple 
structure  of  their  vegetative  organs,  although 
their  reproductive  organs  may,  on  the  other 
hand,  be  very  complex.  For  example,  the 
somewhat  elaborate  objects  popularly  known 
as  "  fungi  "  are  not  the  vegetative  bodies  of 
the  fungi  at  all,  but  only  their  fructifications. 
Flowering  plants  which  have  adopted  the 
non-chlorophyll  habit  similarly  tend  to  be- 
come greatly  modified  in  everything  except 
their  reproductive  organs — the  flowers  and 


162  PLANT  LIFE 

fruit.  We  often  speak  of  them  as  degraded 
or  degenerate  forms,  but  this  is  a  somewhat 
loose  and  inaccurate  form  of  expression.  The 
vegetative  structure  has,  it  is  true,  been 
simplified,  but  in  such  a  way  as  to  render 
the  plants  much  better  adapted  to  the  new 
conditions  of  nutrition  than  they  ever  could 
have  been  had  they  retained  the  complexity 
appropriate  to  the  green  ancestral  type. 

For  the  moment  we  will  pass  over  the 
bacteria,  which  differ  in  many  respects  from 
other  plants,  and  survey  the  most  salient 
features  presented  by  the  fungi. 

Fungi,  like  the  flowering  parasites  above 
mentioned,  have  descended  from  green 
ancestors,  but  it  is  among  the  lower  ranks 
of  the  vegetable  kingdom  that  their  origin  is 
to  be  sought.  It  is  tolerably  certain  that  the 
class  of  fungi,  as  we  know  them  to-day,  repre- 
sent a  number  of  not  very  closely  related 
families,  and  that  these  have  descended  from 
more  than  one  chlorophyllous  algal  stock. 
There  is  no  reason,  therefore,  to  think  that 
their  vegetative  structure  has  undergone 
very  important  alteration  in  the  direction  of 
simplicity.  It  would  probably  be  more  in 
accordance  with  facts  to  say  that  it  has  never 
emerged  from  a  primitively  simple  type  of 
organisation. 

The  body  of  a  young,  actively  growing 
fungus  consists  entirely  of  simple  tubular 
threads,  or  hyphce,  which,  in  the  majority  of 
species,  are  partitioned  by  cross  walls.  These 


THE  FUNGI  163 

hyphse  ramify  over  and  through  the  nutrient 
substratum,  and  sometimes  they  cohere  in 
strands.  They  then  become  easily  visible  to 
the  naked  eye  and  are  popularly  known  as 
"  spawn."  The  spawn  thus  represents  a 
specially  luxuriant  condition  of  the  vegetative 
body  or  mycelium  of  the  fungus,  the  mycelium 
being  taken  as  a  collective  term  for  the  mass 
of  hyphal  threads  of  which  the  vegetative  part 
of  the  fungus  is  composed.  Even  the  "  toad- 
stools "  and  other  fructifications  of  fungi  are 
entirely  produced  by  the  organised  weaving 
of  the  hyphae  into  more  or  less  solid  structures, 
followed  by  differentiation  in  the  mass  thus 
formed,  together  with  the  specialised  growth 
of  certain  groups  of  hyphse. 

The  fungus  obtains  the  whole  of  its  food 
from  the  substratum  in  which  it  is  growing, 
and  some  of  the  nutriment  is  always  of 
organic  origin.  Inasmuch  as  the  fungus 
contains  neither  chlorophyll,  nor  any  other 
material  which  would  enable  it  to  utilise 
the  energy  of  sunlight,  there  is  no  necessity 
for  the  growing  mass  to  expose  itself  to  light 
at  all.  Indeed,  to  do  so  would  carry  with 
it  the  manifest  disadvantages  of  removing  it 
from  the  immediate  source  of  nutrition,  as 
well  as  of  exposing  it  to  the  risk  of  desiccation. 

Now,  having  regard  to  the  fact  that  the 
vegetative  plant  of  the  fungus  is  absorbing 
the  whole  of  its  food  in  a  dissolved  form  from 
the  material  in  which  it  is  ramifying,  it  will 
be  evident  that  the  larger  the  surface,  in 


164  PLANT  LIFE 

proportion  to  the  bulk  of  the  fungus,  the 
better  this  process  of  absorption  will  go  on. 
Consequently,  we  can  easily  understand  that, 
up  to  a  certain  point,  the  simpler  the  structure, 
and  the  more  independent  the  individual 
mycelial  hyphse  are  of  one  another,  the  more 
thoroughly  the  plant  is  adapted  to  explore  its 
nutrient  surroundings  and  to  absorb  its  food. 
As  with  the  root-hairs  of  a  root,  increase  of 
surface  is  the  keynote  of  the  performance. 
We  find  that  all  non-green  plants  tend  towards 
this  adapted  simplicity  of  organisation,  though 
the  higher  green  plants  have  far  to  go  before 
they  can  shake  off  the  shackles  of  complexity. 
It  is  only  from  an  anthropomorphic  stand- 
point, then,  that  we  can  regard  these  plants 
as  merely  degraded  or  degenerate,  for  they 
are  just  as  accurately  adapted  to  obtain 
their  more  specialised  form  of  food  as  is  the 
complex  green  plant  in  relation  to  its  simpler 
sources  of  nutrition. 

Furthermore,  from  this  physiological  point 
of  view,  the  fungi  are  even  more  complex 
than  their  green  ancestors,  for  they  do  not 
merely  absorb,  but  they  also  profoundly 
influence  the  nature  of  the  substratum  in 
which  they  live  by  means  of  the  ferments 
and  other  substances  which  they  excrete. 
Some  of  these  non-green  plants  show  an  almost 
diabolical  ingenuity  of  physiological  action, 
as,  for  example,  when  some  of  the  parasites, 
by  emitting  an  attractive  excretion,  cause 
their  victims  to  actually  grow  towards  them, 


THE  FUNGI  165 

Although  the  fungi,  as  a  class,  are  dependent 
for  their  nourishment  on  substances  which 
have  originated  from  other  living  things, 
they  differ  a  good  deal  among  themselves  as 
to  the  kind  of  food  material  they  can  use. 
Some  are  dependent  on  the  living  bodies  of 
animals  or  plants,  and  then  we  call  them 
parasites.  Others  live  on  dead  or  decaying 
remains,  and  are  commonly  termed  sapro- 
phytes. The  saprophytes  are  a  very  extensive 
class,  and  include  many  species  that  can  live 
on  relatively  simple  organic  residues  such  as 
sugar,  organic  acids,  and  so  forth.  But  these 
simpler  feeders  are  connected  with  more 
obviously  saprophytic  types  by  all  con- 
ceivable intermediate  forms,  and  even  the 
distinction  between  saprophytes  and  parasites 
is  far  from  absolute,  for  many  saprophytes 
can  become  parasites,  and  vice  versa. 

Now  the  utilisation  of  all  food,  regarded  as 
a  means  to  an  end,  is  connected  with  changes 
in  the  states  of  energy.  Complex  organic 
substances  possess  a  considerable  amount  of 
energy  in  a  locked-up,  or  potential  form, 
When  the  food  substances  undergo  oxidation 
and  are  broken  down  into  simpler  ones,  the 
potential  energy  is  set  free  as  kinetic  energy, 
just  as  happens  when  a  piece  of  coal  is 
oxidised  or  burnt.  This  kinetic  energy  is 
directly  available  for  doing  work,  and  may 
be  utilised  to  boil  a  kettle  or  to  build  up  the 
body  of  a  fungus.  In  the  former  case,  the 
kinetic  energy  is  used  to  alter  the  physical 


166  PLANT  LIFE 

state  of  the  water;  in  the  latter,  it  is  used 
to  construct  complex  out  of  relatively  simple 
chemical  substances.  And  in  this  process 
the  energy,  instead  of  becoming  dissipated 
over  an  infinite  field,  is  concentrated  within 
narrow  limits.  It  is  again  locked  up  as 
potential  energy  in  the  construction  of  chemi- 
cal molecules,  and  it  can  be  reconverted  into 
the  kinetic  form  when  the  molecular  groupings 
once  more  break  down. 

We  have  already  seen  that  the  various 
organic  substances  on  which  the  fungi  and 
other  colourless  parasites  and  saprophytes 
subsist  are  all  traceable  more  or  less  directly 
to  the  synthetic  processes  of  the  green  plants. 
The  latter  are  empowered  to  build  up  this 
organic  material  by  utilising  the  energy  of 
the  sunlight  for  the  work.  Thus  we  are 
justified  in  saying  that  the  products  of 
photosynthesis  practically  represent  the  total 
means  available  for  supplying  the  energy 
required  to  drive  the  machinery  of  the  rest 
of  the  life  of  the  world.  In  other  words, 
the  potential  energy  of  the  organic  food  is 
resolved  into  kinetic  energy  in  the  body  of 
an  organism,  and  it  is  solely  by  virtue  of 
this  kinetic  energy  that  an  organisn  can  live 
and  move  and  have  its  being. 

Now  the  essential  chemical  process  which 
is  carried  on  when  the  energy  of  the  sunlight 
drives  the  photosynthetic  machinery  of  the 
green  plants  results  in  reduction  or  deoxida- 
tion.  The  carbohydrate  food — the  tangible 


THE   FUNGI  167 

result  of  this  process — represents  a  store  of 
energy  equal  to  that  required  to  tear  the 
oxygen  away  from  the  various  parts  while  it 
was  in  the  making.  This  energy  can  be 
again  released  in  the  kinetic  form  by  oxida- 
tion. But  it  need  not  take  place  in  one  stage. 
We  shall  get  a  certain  definite  amount  of 
kinetic  energy  set  free  if  we  burn  a  pound  of 
sugar,  but  we  can  break  up  the  sugar  more 
gradually,  and  at  each  stage  a  definite  amount 
of  energy  will  be  liberated.  It  is  only  when 
we  have  completed  the  destructive  process, 
and  carbon  dioxide  and  water  alone  remain, 
that  we  can  get  no  more  energy  from  our 
sugar  products.  And  if  we  were  to  add  up 
the  various  amounts  of  energy  liberated 
during  the  various  stages  of  destruction  of  the 
sugar,  they  would  amount  to  the  same  figure 
as  if  we  had  burnt  (i.  e.  oxidised)  it  at  once. 

Of  course,  neither  an  animal  nor  plant  can 
ever  burn  itself  completely  down  to  the  simple 
substances  into  which  it  is  capable  of  being 
resolved.  Some  portions  of  it  will  remain 
intact,  and  of  the  rest  all  sorts  of  intermedi- 
ate products  will  be  formed.  These  possess 
different  energy  values  according  to  their 
composition,  and  especially  according  to  their 
complete  or  incomplete  state  of  oxidation. 

It  is  on  these  intact  or  partially  broken  up 
chemical  substances  that  the  non-green  organ- 
isms are  able  to  live,  and  the  fungi  and  bacteria 
especially  serve  an  important  purpose  in  the 
world  because  they  are  able  to  induce  the 


168  PLANT  LIFE 

breaking  up  of  the  complex  substances 
constituting  the  dead  bodies  of  other  organ- 
isms into  simpler  chemical  compounds,  and 
sometimes  even  into  their  elements.  They 
utilise  a  certain  amount  of  the  energy  thus 
finally  set  free  in  building  up  the  materials 
of  which  their  own  bodies  are  constructed, 
but  the  proportion  of  matter  (and  of  energy 
too)  thus  locked  up  again  is  extremely  small 
when  compared  with  that  which  has  been 
liberated  in  the  process. 

Regarded  from  this  standpoint  the  sun  is 
seen  to  be  the  very  fountain  of  life  and  power. 
The  complex  materials  which  jostle  each  other 
as  they  are  borne  down  the  stream  of  life 
break  down  here,  provide  the  power  for 
synthesis  there,  but  on  the  whole  are  ever 
losing  more  of  their  energy,  for  some  is 
continually  dissipated  as  heat.  Many  of  the 
useless  molecular  fragments  which  drift  out 
of  the  stream  of  life  are  presently  caught  up 
again  at  the  fountain-head,  the  chlorophyll 
machinery  driven  by  the  solar  energy  once 
more  draws  them  into  the  mill,  there  they 
are  broken  up,  are  compounded  with  other 
ingredients,  and  the  whole  turned  out  once 
more  as  energy-containing  carbohydrate. 
Again  the  compounded  matter  re-enters  the 
stream  of  life,  to  part,  little  by  little,  with 
the  energy  it  contains.  Some  is  destined  to 
reappear  in  vital  units  of  almost  infinite 
complexity,  the  rest  is  utilised  or  lost  in 
various  ways,  and  the  broken  and  buffeted 


THE  FUNGI  169 

fragments  of  the  carbohydrate  are  once  more 
cast  forth  into  the  inorganic  world  as  simple 
and  vitally  useless  molecules. 

Let  us  take  a  specific  instance  by  way  of 
illustration  of  the  foregoing  consideration. 
Wood  is  an  almost  imperishable  substance,  at 
least  under  ordinary  circumstances,  and  so 
long  as  it  is  preserved  from  the  attacks  of 
living  organisms.  But  timber  is  liable  to  the 
depredations  of  a  large  number  of  different 
fungi  which,  under  conditions  favourable  to 
their  existence,  are  able  to  use  it  as  food. 
They  act  on  it  by  various  ferments,  bringing 
some  of  its  constituents  into  solution,  absorb- 
ing and  partially  breaking  them  down.  The 
wood  is  soon  reduced  to  a  friable  mass, 
weighing  far  less  than  the  original  timber,  owing 
to  the  decomposition  of  its  chemical  substance, 
and  the  elimination  of  some  of  the  products 
of  its  oxidation.  Even  the  solid  residue  will 
soon  disappear  under  the  further  influence 
of  a  succession  of  micro-organisms,  which  will 
finally  disintegrate  whatever  the  fungus  may 
have  left.  Thus,  in  course  of  time,  the  whole 
of  the  chemical  materials  out  of  which  the 
timber  was  constructed  will  again  become 
part  of  the  floating  capital  of  nature,  available 
for  the  constructive  processes  of  new  organ- 
isms, or  destined  for  yet  other  purposes  in  the 
chemical  change  going  on  in  the  world. 

Most  timbers  are  liable  to  infection  by 
fungi  when  they  are  stored  in  a  damp  condi- 
tion. The  danger  is  greatly  accentuated  if 


170  PLANT  LIFE 

the  wood  contains  organic  matter  which  is 
either  soluble  or  easily  rendered  so,  and  this 
is  one  of  the  reasons  why  trees  are  felled  at 
a  time  of  the  year  in  which  the  wood  naturally 
contains  least  moisture  or  sap,  and  why  the 
tree  is,  or  should  be,  left  to  "  season  "  (i.  e.  to 
dry)  before  being  cut  up.  A  source  of  danger 
to  stored  timber  may  arise  from  contamina- 
tion with  organic  matter  consequent  on  the 
neglect  of  proper  sanitary  precautions.  For 
wood  which  is  so  contaminated  forms  a  very 
suitable  substratum  for  the  germination  of 
a  number  of  pests,  and  attacks  of  the 
dangerous  "  dry  rot  "  fungus  (Merulius)  has 
sometimes  been  traced  to  this  source.  When 
once  a  wood- destroy  ing  mycelium  has  estab- 
lished itself  in  a  piece  of  timber  it  may  be 
difficult  to  get  rid  of  it.  It  will  often  lie 
dormant  for  a  considerable  time  when  the 
wood  is  dry,  and  only  moisture  or  dampness 
is  required  to  awaken  it  to  dangerously  active 
growth. 

The  wood  of  living  trees  is  liable  to  attack 
by  various  fungi  which  commonly  gain  access 
to  it  by  means  of  wounds,  due  to  abrasion  of 
bark  or  the  falling  off  of  branches.  The 
mischief  is  usually  far  advanced  by  the  time 
the  first  symptoms  are  apparent,  and  it  is 
often  then  too  late  to  adopt  remedial  measures. 
In  these  matters  "  a  stitch  in  time  saves 
nine,"  and  it  is  generally  a  simpler  matter 
to  clean  and  tar  a  wound  at  once  so  as  to 
prevent  the  entrance  of  the  disease-producing 


THE  FUNGI  171 

organism  rather  than  to  endeavour  to  extir- 
pate it  afterwards. 

Some  of  these  destructive  fungi,  instead  of 
only  attacking  the  dead  tissues — the  wood — 
of  the  trees,  invade  and  kill  the  living  cells. 
This  is  a  more  serious  matter,  for  we  must 
remember  that  only  a  very  small  part  of  a 
trunk  is  really  alive  in  the  strict  sense  of  the 
word,  that  is,  contains  living  protoplasm. 
Any  pest  which  attacks  the  living  tissues,  and 
especially  the  cambium,  often  speedily  kills 
it,  or  at  any  rate  renders  it  practically  worth- 
less. Such  fungi  are  those  which  produce 
the  larch  canker  (Dasyscypha  Willkommii)  and 
the  beech  canker  (Nectria  ditissima),  the 
latter  being  especially  destructive  to  the  cells 
of  the  cambial  region  and  thus  producing 
very  dangerous  lesions. 

One  of  the  most  interesting  of  the  tree 
diseases  is  that  produced  by  the  fungus  known 
as  Armillaria  mellea.  The  fructifications  are 
easily  recognised  as  clusters  of  brown  toad- 
stool-like bodies  which  spring  from  the  roots 
or  stumps  of  dead  trees  at  the  ground  level. 
Each  "  toadstool "  is  characterised  by  the 
possession  of  a  ring  or  frill  underneath  the 
cap  which  bears  the  gills.  Before  its  life 
history  became  known,  and  consequently 
methods  could  be  devised  to  check  its  progress, 
the  fungus  was  a  very  dangerous  one,  especi- 
ally when  it  invaded  the  pine  woods,  for  it 
spreads  fairly  rapidly  from  one  tree  to 
another.  The  mycelium  grows  in  the  cambial 


172  PLANT  LIFE 

region,  and  also  in  the  sapwood  of  the  tree, 
but  the  special  point  of  interest  about  this 
fungus  lies  in  the  circumstance  that  black 
cord-like  mycelial  strands  are  produced  in  the 
tree  by  the  approximation  of  hyphse,  which 
then  become  woven  into  thin  strands.  These 
grow  as  organised  structures,  and  some  of 
them  force  their  way  out  of  the  tree  below 
the  surface  of  the  soil,  there  continuing  to 
elongate  till  they  reach  the  roots  of  other 
pines.  They  then  enter  these,  and  so  the 
pest  may  easily  assume  the  character  of  an 
epidemic,  extending  from  one  pine  to  another 
as  from  a  centre,  and  killing  the  trees  in  its 
advance. 


CHAPTER  XV 

FUNGAL    PARASITES 

THE  history  of  our  cultivated  plants,  both 
in  Europe  and,  to  a  far  greater  extent, 
in  the  tropics,  bears  abundant  testimony  to 
the  magnitude  of  the  evils  caused  by  fungal 
enemies.  The  conditions  under  which  crops 
are  generally  grown  happen,  unfortunately,  to 


FUNGAL  PARASITES  173 

be  specially  favourable  to  the  spread  of 
infectious  fungal  disease.  Diseases  of  this 
kind  are  apt  very  easily  to  get  beyond  control 
unless  they  can  be  checked  in  their  early 
stages.  Even  with  all  our  present  precautions 
the  annual  loss  from  fungal  disease  is  gigantic, 
amounting  to  many  millions  sterling  in  this 
country  alone. 

Regarded  from  a  biological  point  of  view, 
the  parasitic  species  are  in  many  respects 
the  most  interesting  of  the  group  of  fungi. 
In  spite  of  their  simple  structure  we  find 
their  physiological  properties  are  very  much 
specialised,  and  admirably  adapted  to  their 
particular  habits  as  parasites.  In  these 
respects,  however,  they  show  wide  differences 
in  behaviour.  Some  ruthlessly  kill  their  host, 
reducing  it  to  a  mass  of  rottenness — for 
instance,  the  Phytophthora,  which  is  the 
cause  of  potato  disease.  Others,  while  taxing 
their  host  for  their  own  means  of  support, 
make  no  excessive  demands,  and  may  even 
stimulate  a  locally  increased  growth  on  the 
part  of  their  hosts,  at  any  rate  during  the 
earlier  stages  of  their  development.  Some 
of  the  rust  fungi  furnish  examples  of  this, 
causing  local  thickenings  on  the  stems  of 
roses,  nettles  and  other  plants.  A  very 
striking  instance  of  the  influence  that  a 
parasite  may  exert  on  its  host  is  afforded  by 
a  species  of  smut  (Ustilago  violacea)  which 
sometimes  infests  the  Red  Campion  (Lychnis 
dioica)  of  the  hedgerows. 


174  PLANT  LIFE 

This  species  of  Lychnis  is  a  dioecious  plant. 
That  is  to  say,  the  flowers  of  some  plants  are 
exclusively  female  whilst  the  rest  are  ex- 
clusively male.  The  unisexual  character  is 
produced  by  the  abortion  of  the  pistil  in  the 
flowers  of  the  male,  and  of  the  stamens  in 
those  of  the  female  plants.  The  fungus  only 
reaches  maturity  and  produces  its  violet 
powdery  spores  in  the  stamens.  So  far  as 
the  male  plants  are  concerned  there  is  no 
difficulty,  but  with  the  female  flowers  it  is 
otherwise.  What  the  fungus  does  when  it 
attacks  the  latter  is  to  stimulate  the  plant  to 
produce  stamens  exactly  like  those  of  a  male 
flower.  The  mycelium  grows  sparsely  in 
them  until  the  pollen  sacs  are  approaching 
maturity,  then  it  suddenly  breaks  out  into 
virulence  and  destroys  the  pollen-producing 
tissues,  filling  up  the  space  with  its  own  spores. 
Nor  does  the  influence  of  this  remarkable 
parasite  stop  here,  for  the  pistil  is  arrested 
at  an  early  stage  of  its  development  and  in 
certain  other  structural  characters  the  flower 
closely  approximates  to  that  characteristic 
of  a  male  plant — so  closely  indeed  that  very 
careful  experiments  were  needed  to  clear  up 
the  matter. 

It  is  not  known  what  the  substance  formed 
by  the  fungus,  and  responsible  for  the  change, 
really  is.  All  attempts  to  imitate  its  action, 
and  to  produce  a  similar  result  artificially 
have  so  far  proved  unsuccessful. 

The  somewhat  common  "  witches'-brooms  " 


FUNGAL  PARASITES  175 

furnish  another  example  of  remarkable  inter- 
ference with  the  ordinary  growth-processes 
of  the  host  plant  that  a  parasite  is  able  to 
induce.  The  wild  cherry  trees  are  particu- 
larly subject  to  the  attacks  of  a  fungus 
(Exoascus)  which  fruits  within  the  leaves 
and  alters  the  boughs  affected  by  it  in  a 
curious  manner.  They  are  much  more  freely 
branched,  the  leaves  are  often  smaller  and 
sometimes  deformed,  and  flowers  are  seldom 
or  never  produced  on  the  affected  parts. 
Another  kind  of  witches'-broom  occurs  on  the 
fir  trees  in  continental  forests,  though  they 
are  not  so  frequently  seen  in  this  country. 
It  is  produced  by  one  of  the  rust  fungi 
(JEcidium  elatinum).  A  twig  affected  with  it 
is  a  striking  object,  inasmuch  as  it  grows  up 
vertically  on  the  bough  instead  of  horizontally. 
This  erect  habit  is  maintained,  and  as  the 
years  pass  the  witches'-broom  comes  to 
resemble  a  little  Christmas  tree  arising  from 
an  ordinary,  horizontally-growing  bough  of 
the  fir  tree.  Several  kinds  of  firs  are  liable 
to  the  attacks  of  this  fungus,  but  it  is  on  the 
silver  fir  that  the  witches'-broom  is  most  often 
seen. 

Such  relations  between  fungus  and  host  as 
those  just  described,  and  many  other  examples 
might  be  added,  very  clearly  prove  that  these 
apparently  simple  parasites  are  remarkably 
complicated  from  a  physiological  point  of  view. 
The  surprising  thing  about  them  is  their  very 
accurate  degree  of  specialisation  to  the  hosts, 


176  PLANT  LIFE 

by  which  they  have  been  enabled  easily  to 
bring  about  these  quite  definite  and  charac- 
teristic changes  of  form.  It  is  evident  that 
the  physiological  adjustments  must  be  very 
delicate,  for  all  attempts  to  imitate  them  have 
so  far  ended  in  failure.  But  it  is  just  on  this 
quality  that  the  more  "  educated "  fungal 
parasites  depend  for  their  subsistence,  and 
it  is  a  quality  which  they  share  with  other 
specialised  vegetable  parasites,  as  well  as  with 
the  gall-producing  animals. 

It  has  already  been  said  that  all  stages  can 
be  traced  between  a  saprophytic  and  a  para- 
sitic habit  as  exemplified  in  the  life  histories 
of  different  fungi.  Sometimes  it  is  possible 
to  trace  the  change  from  one  to  the  other  in  a 
single  individual.  This  may  occur  either  by 
the  fungus  acquiring  additional  powers  of 
attack  or  it  may  happen  through  a  diminution 
of  the  power  of  resisting  infection  on  the  part 
of  the  victim. 

As  an  example  of  the  first  of  these  we  may 
select  a  common  brown  mould  known  as 
Botrytis  cinerea.  Like  many  of  these  fungi, 
Botrytis  represents  the  mould  form  of  one  of 
the  cup  fungi  (Peziza). 

If  the  spores  of  the  peziza  fructification  are 
sown  on  a  living  plant,  say  a  carrot,  they 
usually  fail  to  infect  it ;  but  if  they  happen  to 
fall  on  to  a  dead  or  decaying  portion  of  the 
carrot,  they  grow  and  produce  a  mycelium 
which  spreads  through  the  dead  tissues. 
And  this  mycelium  can  now  invade  the  living 


FUNGAL  PARASITES  177 

plant.  The  hyphae  secrete  a  poison  which 
kills  the  cells  in  advance  of  its  track,  and  thus 
the  fungus  succeeds  in  completely  destroying 
the  plant.  Critically  regarded,  the  change 
from  saprophytism  to  parasitism  in  this 
instance  is  a  somewhat  imperfect  one  because 
the  fungus,  inasmuch  as  it  kills  in  advance, 
is  really  living  on  dead  tissues.  But  it  shares 
this  property  with  the  majority  of  the  de- 
structive parasites.  It  is  only  the  more 
specialised  forms  that  tax  but  do  not  destroy. 
It  represents  a  transitional  phase,  and  one  of 
interest,  inasmuch  as  it  shows  how  appropriate 
nourishment  may  accelerate,  and  increase 
to  an  effective  degree,  physiological  powers 
already  present,  but  normally  inadequate, 
for  purposes  of  direct  application. 

As  regards  the  susceptibility  of  the  host 
plant  to  fungal  attacks,  it  is  a  matter  of  the 
commonest  experience  that  some  individuals 
of  a  race  are  more  liable  to  contract  disease 
from  these  causes  than  others.  Every  year 
sees  the  introduction  of  new  varieties  of 
potatoes  which  are  claimed  to  be  immune 
towards  the  disease  (Phytophthora)  that  often 
does  so  much  damage  to  the  crop.  Sometimes 
these  varieties  are  resistant  in  certain  districts 
and  less  so  in  others,  and  it  may  happen  that 
their  immunity  gradually  disappears  after 
some  years  of  cultivation.  It  is  evident,  then, 
that  immunity  in  such  instances  is  not  a 
simple  matter.  Whilst  it  may  partially  depend 
on  those  properties  which  together  make  up 


178  PLANT  LIFE 

the  "  constitution "  of  a  plant,  it  is  also 
affected  by  the  influence  of  surrounding 
conditions  of  life. 

We  know  very  little,  as  yet,  about  the 
nature  of  "  constitutional "  resistance.  In 
some  cases  it  depends  on  a  well-developed 
epidermis,  and  on  the  absence  of  attractive 
substances,  such  as  sugar,  from  parts  of  the 
plant  readily  accessible  to  the  fungus.  A 
curious  example  of  immunity  against  rust 
fungi  is  furnished  by  some  of  the  cereals 
recently  raised  at  Cambridge.  The  fungus 
normally  gains  access  to  the  interior  of  the 
leaf  by  the  germ  tube  or  hypha  growing  in 
by  way  of  a  stoma,  and  then  attacking  the 
living  cells.  But  it  is  possible  to  find  a 
wheat  plant  so  sensitive  to  the  influence  of 
the  fungus  that  the  cells  die  immediately  the 
hypha  approaches  them.  The  fungus  is  thus 
effectively  starved,  and  is  unable  further  to 
infect  the  plant.  It  may  also  happen  that 
when  a  fungal  hypha  has  entered  a  plant,  the 
attacked  and  injured  tissues  are  cut  off  from 
communication  from  the  healthy  ones  by 
a  layer  of  impervious  cork,  and  in  this  way 
the  further  spread  of  the  disease  within  the 
body  of  the  plant  is  prevented. 

The  part  played  by  the  environment  in 
increasing  liability  to  infection  depends  on  a 
number  of  possible  factors,  all  of  general 
biological  interest.  A  close  damp  atmosphere 
is  not  only  favourable  to  the  germination  of 
the  fungus  spore,  but  it  may  at  the  same  time 


FUNGAL  PARASITES  179 

injuriously  affect  the  development  of  the 
cuticle  of  the  leaf.  Or,  again,  it  may  lead  to 
an  excessive  amount  of  watery  sap  in  the 
superficial  tissues,  quite  apart  from  the  effects 
of  external  moisture  on  the  outer  surfaces  of 
the  stems  and  leaves.  It  is  well  known  that 
bad  cultural  conditions  may  predispose  plants 
to  disease,  and  observation  teaches  that  some- 
times, at  any  rate,  the  effects  are  due  to 
imperfect  development  of  the  tegumentary 
tissues. 

The  presence  of  nitrogenous  manure  in 
excessive  quantities,  in  proportion,  that  is, 
to  the  other  nutritive  constituents  of  the  soil, 
is  another  predisposing  cause  of  fungal  attack. 
It  operates  in  several  ways,  but  often  in- 
directly by  causing  an  undue  accumulation 
of  soluble  nutritious  substances  in  tissues 
and  cells  the  walls  of  which  are  imperfectly 
thickened. 

Starvation  of  an  essential  food  constituent 
may  act  as  a  specific  cause  of  predisposition. 
Thus  many  grasses,  when  they  are  grown  on 
land  in  which  the  supply  of  potash  salts  is 
inadequate,  become  very  liable  to  epidemic 
attacks  of  a  fungus  known  as  Epichloe  typhina. 
The  disease  makes  its  appearance  in  the  form 
of  white  (changing  to  yellow)  zones  situated 
just  above  the  knots  of  the  stem,  and  extend- 
ing upwards  for  a  centimeter  or  two.  These 
zones  mark  the  regions  where  the  reproductive 
organs  of  the  fungus  are  formed.  A  poor 
supply  of  potash  is  also  known  to  affeet  the 


180  PLANT  LIFE 

formation  of  carbohydrates  in  the  plant. 
In  other  words,  the  predisposition  to  infec- 
tion in  this  instance  is  probably  connected 
with  a  disturbance  of  the  photosynthetic 
processes. 

The  whole  matter  of  immunity  is  evidently 
very  closely  related  with  nutrition.  It  may 
be  the  result  either  of  a  defective  hereditary 
constitution  or  of  some  property  of  the 
environment  (e.  g.  excessive  or  deficient  sup- 
plies of  essential  food  elements)  which  inter- 
feres with  the  chemical  processes  of  the 
manufacture,  distribution,  or  utilisation  of 
food  within  the  organism.  The  part  played 
by  the  fungus  depends  on  its  physiological 
capability  to  take  advantage  of  the  host 
plant.  It  must  be  able  to  enter  the  body  of 
its  victim,  and  either  utilise  there  whatever 
stores  of  nutriment  are  directly  available,  or 
it  must  modify  the  vital  processes,  and  in 
this  way  secure  for  itself  the  nourishment  it 
needs.  Some*  of  the  extremely  specialised 
parasites,  and  especially  some  of  the  rusts,  are 
limited  to  particular  species,  and  even  sub- 
species, of  plants  as  hosts.  This  can  only  be 
interpreted  as  meaning  that  they  are  adapted 
to  live  on  a  very  special  kind  of  food,  and 
perhaps  also  that  they  are  easily  affected  in  a 
prejudicial  manner  by  substances  which  occur 
in  species  nearly  related  to  their  own  proper 
hosts.  But  even  the  specialised  parasites 
are  capable  of  further  extending  their  range. 
For  example,  the  Brome  grasses,  of  which 


FUNGAL  PARASITES  181 

there  are  a  number  of  species  in  Britain,  are 
liable  to  the  attacks  of  a  parasitic  rust  fungus 
known  as  Puccinia  dispersa.  Now  when  the 
parasite  has  been  growing  for  a  while  on  one 
species  of  brome,  it  loses  the  power  of  infecting 
some  of  the  others.  And  yet  the  puccinia 
is  found  flourishing  on  these  apparently 
resistant  species  also.  The  clue  to  the  puzzle 
lies  in  the  fact  that  although  the  puccinia 
thus  develops  "  races "  which  preferably 
attack  single  species  of  brome,  they  can  be 
induced  to  recover  their  powers  of  infecting 
others  by  the  simple  device  of  cultivating 
them  on  other  species  which  are  only  inter- 
mediate in  ttheir  powers  of  resistance.  Thus 
a  race  which  will  thrive  on  a  species  A,  but 
cannot  attack  another  species  C,  will  never- 
theless recover  the  power  of  doing  so  if  it  be 
grown  on  a  third  species  B.  This  remarkable 
occurrence  of  "  bridging  species  "  of  plants  is 
of  obvious  importance  in  connection  with  the 
sudden  appearance  of  parasitic  epidemics. 
It  is  not  confined  to  the  rust  fungus,  but  is 
known  to  extend  to  others ;  amongst  them  is 
Erysiphe  graminis,  which  also  infests  the 
brome  grasses 


182  PLANT  LIFE 


CHAPTER  XVI 

FLOWERING  PARASITES 

IT  has  been  already  pointed  out  that  the 
non-green  saprophytes  or  parasites  are  by 
no  means  limited  to  the  classes  of  Fungi  and 
Bacteria.  Quite  a  large  number  of  the 
flowering  plants  have  adopted  the  habit 
of  utilising  extraneous  stores  of  organic 
food,  and  in  connection  therewith  have  more 
or  less  lost  the  faculty  of  producing  chloro- 
phyll. There  is  the  strongest  possible 
evidence  that  the  change  has  come  about 
in  correlation  with  the  altered  conditions  of 
nutrition.  In  other  words,  the  more  or  less 
complex  food-substances  present  in  the  living 
or  dead  bodies  of  other  organisms  do  influence 
the  structure  of  those  plants  which  make  use 
of  them,  and  one  result  is  seen  in  the  loss  of 
the  faculty  of  producing  chlorophyll. 

One  might,  then,  expect  to  find  many  links 
connecting  the  normal  green  plants  with  those 
highly  specialised,  or  as  they  are  often  called, 
degraded,  forms  characteristic  of  extreme 
parasites.  And  as  a  matter  of  fact  we  can 
trace  such  a  series  in  a  number  of  instances. 

The  Mistletoe  (Viscum  album)  is  a  parasite 
which  betrays  very  little  of  the  degeneration 


FLOWERING  PARASITES         183 

we  often  associate  mentally  with  a  parasitic 
habit,  but  it  has  nevertheless  undergone 
considerable  modification  in  its  root  structure, 
whilst  there  is  little  in  its  stems  and  leaves, 
or  in  the  internal  anatomy  of  these  organs, 
to  indicate  its  particular  habit  of  life.  The 
reason  lies  solely  in  the  circumstance  that  it 
has  in  no  way  abandoned  the  functions  of 
independent  photosynthesis.  It  only  with- 
draws water  and  inorganic  salts  from  the 
host  plant  which  it  infests,  but  makes  no 
demand  upon  it  for  sugars  and  other  com- 
plex organic  food.  It  is  mainly  in  respect  of 
its  root  system  that  it  has  become  modified, 
for  the  machinery  requisite  for  continual 
absorption  of  water  from  the  wood  of  a  living 
tree  is  very  different  from  that  which  is 
adapted  to  discharge  a  similar  function  in 
the  soil.  Branching  green  structures,  which 
probably  represent  creeping  stems,  traverse 
the  rind  of  the  tree  on  which  the  mistletoe  is 
growing,  and  from  these  there  grow1  peg-like 
protuberances  which  become  firmly  embedded 
in  the  wood.  These  pegs  are  the  real  mistletoe 
roots,  and  they  are  very  carefully  adjusted  in 
the  manner  of  their  growth  to  the  habits  of 
the  particular  tree  in  which  they  occur.  Their 
rate  of  elongation  exactly  coincides  with  that 
of  the  increase  in  thickness  of  the  branch. 
It  is,  of  course,  only  this  accurate  adjustment 
that  renders  it  possible  for  the  mistletoe  to 
flourish  at  all,  for  it  is  clear  that  the  roots 
would  otherwise  be  unable  to  maintain  that 


184  PLANT  LIFE 

intimate  connection  with  the  wood  of  the 
tree  which  is  necessary  both  to  fix  the  parasite 
to  its  support  and  to  draw  from  the  host 
plant  the  supplies  of  water  it  requires  for  its 
own  purposes. 

There  are  other  near  relatives  of  the 
mistletoe,  belonging  to  the  genus  Loranthus, 
which  are  far  more  dangerous  and  destructive 
parasites.  These  plants  are  common  in  the 
tropics,  and  they  form  leafy,  bush-like  growths 
in  the  trees  they  infest.  Many  of  them  bear 
beautiful  trusses  of  red  flowers,  and  they  some- 
what recall  the  appearance  of  fuchsia  bushes 
perched  among  the  trunks  and  boughs  on  the 
outskirts  of  the  forest. 

Like  the  mistletoe,  it  is  the  roots  of  a  loran- 
thus  that  have  undergone  important  changes 
in  relation  to  the  parasitic  habit.  They  arise 
as  sucker-like  outgrowths  from  special  creep- 
ing stems  of  the  loranthus  which  grow  along 
the  surface  of  the  tree.  As  the  sucker-bearing 
branches  are  freely  produced,  and  may  reach 
a  considerable  length,  the  parasite  often  does 
very  serious  damage. 

It  is  not  a  little  curious  that  in  a  large 
family  of  plants  like  the  Loranthaceae,  to  which 
both  Loranthus  and  the  mistletoe  belong,  some 
species  should  not  have  advanced  still  farther 
in  the  parasitic  direction.  But  although 
nearly  all  of  them  draw  their  water  supplies 
from  another  plant,  they  have  never  taken 
the  final  step  of  absorbing  from  it  the  organic 
food.  They  have  consequently,  or  perhaps 


FLOWERING  PARASITES        185 

one  should  say  correlatively,  retained  their 
leaves,  and  all  the  complexity  of  structure 
which,  as  we  have  seen,  the  presence  of  the 
green  leaf  entails. 

The  parasitic  habit  has  appeared  inde- 
pendently in  a  number  of  other  families  of 
flowering  plants.  In  some  of  them  it  is  char- 
acteristic of  practically  all  the  members, 
just  as  in  the  Loranthaceae  mentioned  above. 
As  a  matter  of  fact,  in  very  many  of  the 
larger  natural  orders  or  families  we  also  find 
species  which  have  more  or  less  broken  away 
from  the  ranks  of  typical  green  plants  in  con- 
nection with  their  assumption  of  saprophytic 
or  parasitic  habits.  Sometimes  we  can 
construct,  within  the  limits  of  nearly  related 
groups,  all  the  stages,  starting  from  a  sort 
of  dalliance  with  robbery  which  is  hardly 
betrayed  by  any  essential  structural  change, 
but  culminating  in  species  which,  so  far  as 
their  vegetative  structure  is  concerned,  have 
lost  all  resemblance  to  the  forms  of  higher 
plants. 

Thus  in  the  alliance  or  family  to  which 
the  snapdragon  belongs,  the  familiar  little 
Eye-bright  (Euphrasia),  abundant  on  grassy 
downs,  the  pink  Lousewort  (Pedicularis)  of 
the  marshes,  and  the  yellow  Cow-wheat 
(Melampyrum)  of  the  woods,  all  have  begun 
to  supplement  the  legitimate  stock  of  food 
which  they  manufacture  for  themselves  by 
stealing  from  adjacent  plants.  This  they  are 
enabled  to  do  owing  to  the  ability  they  possess 


186  PLANT  LIFE 

of  modifying  certain  rootlets  to  form  suckers, 
which  then  become  attached  to,  and  penetrate 
the  tissues  of,  the  underground  parts  of  neigh- 
bouring plants.  Although  they  have  thus 
taken  a  considerable  step  on  the  road  to 
parasitism,  they  are  still  not  very  dependent 
on  the  advantage  they  have  gained.  They 
retain  their  green  leaves,  and  they  will  often 
continue  to  grow  even  when  there  are  no  suit- 
able hosts  which  they  can  attack. 

There  are  other  species,  not  very  distantly 
related  to  the  foregoing,  which  have  advanced 
much  further.  The  Broomrapes  (Orobanche) 
consist  of  a  number  of  species,  each  parasitic 
on  some  kind  of  flowering  plant.  One  of  the 
common  species  grows  on  the  roots  of  the 
broom,  but  it  betrays  no  obvious  sign  of  its 
existence  until  the  flowering  shaft  is  formed. 
The  vegetative  part  of  the  plant  consists  of 
a  small  tuberous  mass  which  is  closely  ad- 
herent to  the  root  of  the  broom^from  which, 
by  means  of  its  sucker-like  roots,  it  derives 
the  whole  of  its  food.  The  bodily  structure 
is  simplified,  and  the  broomrape  consists  of 
little  more  than  a  small  underground  tuber 
which  produces  a  few  specialised  roots.  It 
only  shows  itself  above  ground  when  the 
time  comes  for  it  to  put  forth  its  large  and 
rapidly  developing  flowering  shaft,  on  which 
are  borne  the  reddish  flowers  and  small 
brownish-yellow  leaves.  Its  tiny  seeds, 
like  those  of*  some  other  parasites,  are  re- 
markable in  that  they  do  not  even  begin 


FLOWERING   PARASITES         187 

to  germinate  unless  they  happen  to  lie  in 
close  proximity  to  the  host  of  a  plant  which 
they  can  successfully  attack.  This  striking 
peculiarity  enables  us  to  appreciate  some- 
thing of  the  remarkable  qualities  which 
render  so  specialised  a  parasitic  habit  feasible 
at  all.  For  the  parasite  has  evidently  become 
sensitive  to  the  presence  of  a  definite  sub- 
stance which  emanates  from  the  host-root. 
The  seed  is  then  stimulated,  and  it  awakes 
from  its  dormant  condition.  It  germinates 
and  its  roots  immediately  grow  towards, 
and  penetrate,  the  plant  from  which  it  will 
ultimately  draw  practically  the  whole  of  its 
food. 

A  further  state  of  simplification  of  vege- 
tative structure  is  exhibited  by  Rafflesia 
Arnoldii,  which  is  in  many  respects  perhaps  the 
most  wonderful  of  all  living  flowering  plants. 
It  occurs  in  the  Eastern  tropics,  and  it  pro- 
duces the  largest  flower  known,  for  it  may 
attain  to  as  much  as  a  yard  in  diameter. 
The  rafflesias  are  mostly  parasitic  on  vine- 
like  climbers  (Cissus),  and  they  pass  their 
vegetative  life  entirely  within  the  com- 
paratively slender  stems  of  their  hosts.  In 
this  stage  Rafflesia  is  extremely  simple  in 
structure,  and  indeed  it  resembles  colourless 
fungal  hyphae  more  than  anything  else. 
The  filamentous  cells  branch  through  the  tis- 
sues of  their  host,  and  it  is  only  when  the 
period  of  flowering  draws  near  that  the  para- 
site gives  any  sign  of  what  is  about  to  issue 


188  PLANT  LIFE 

from  the  vine.  The  filamentous  strands  in- 
crease at  the  spot  where  a  flower  is  to  develop, 
and  a  sort  of  ball  of  tissue  is  formed  which 
presently  bursts  through  the  rind  of  the  Cissus 
stem.  Presently  the  ball  splits  open,  and 
there  grows  out  from  within  it  a  flower  bud 
which  opens  out  into  the  single  enormous 
blossom. 

A  more  familiar  flowering  parasite  is  the 
Dodder.  This  plant  infests  various  hosts, 
e.  g.  flax,  clover,  nettles,  gorse,  etc.  It 
rather  suggests  in  appearance  bundles  of 
pink  string  thrown  at  random  over  the  vege- 
tation. It  belongs  to  the  convolvulus  family, 
and  still  more  or  less  retains  the  twining  habit 
so  characteristic  of  many  of  its  relatives.  But 
whereas  the  leafy  convolvulus  merely  supports 
itself  by  twining  round  its  support,  the  almost 
leafless  dodder  puts  forth  suckers  where  its 
stem  is  in  contact  with  that  of  its  host,  and 
from  the  central  portion  of  each  sucker  a 
growth  is  formed  which  penetrates  the  plant. 
In  this  manner  the  dodder  obtains  the  whole 
of  its  food,  both  water  and  organic  substance. 

Although  the  dodder  is  really  little  more 
than  a  specialised  twining  convolvulus,  never- 
theless, in  relation  to  its  parasitic  habit,  it  has 
ceased  to  form  green  leaves,  and  it  is  not  even 
rooted  in  the  soil.  It  is  true  that  when  the 
seed  first  germinates  it  is  anchored  by  hairs 
to  the  ground,  but  the  lower  part  of  the  stem 
soon  dies  away,  and  the  whole  plant  comes  to 
be  absolutely  dependent  on  a  parasitic  life. 


COMPOUND  ORGANISMS         189 

Nevertheless  it  has  not  wholly  lost  its  chloro- 
phyll, and  it  is  of  special  interest  to  find  that 
if  it  happens  to  grow  where  it  cannot  obtain 
plenty  of  nourishment  from  its  host  plant, 
a  larger  amount  of  chlorophyll  can  be 
formed ;  the  stems,  indeed,  may  even  become 
distinctly  green.  Such  an  observation  as 
this  clearly  indicates  how  closely  the  forma- 
tive processes  of  a  plant  are  bound  up  with  its 
nutrition.  But  the  extreme  readiness  with 
which  the  dodder  responds  to  an  appropriate 
stimulus,  by  the  production  of  suckers,  is 
shown  by  the  fact  that  if  one  of  the  stems  of 
the  parasite  happens  to  twine  round  another 
one,  they  commonly  pierce  one  another  with 
the  suckers  which  are  immediately  produced 
at  the  places  of  contact. 


CHAPTER  XVII 

COMPOUND   ORGANISMS 

IT  would  be  an  error  to  imagine  that  all 
the  flowering  plants  in  which  the  production 
of  chlorophyll  is  arrested  are  therefore  to  be 


190  PLANT  LIFE 

regarded  at  once  as  parasites.  We  have 
already  seen  that  vast  numbers  of  the  fungi 
feed  on  dead  remains,  rather  than  on  living 
plants  and  animals.  There  are,  likewise,  many 
flowering  plants  which  apparently  behave  in 
a  similar  manner  and  they  are  generally,  on 
that  account,  classed  as  saprophytes.  But, 
as  we  shall  see,  a  closer  examination  of  the 
facts  indicates  that  many  of  them  more 
nearly  resemble  the  parasites  after  all,  though 
the  method  of  their  parasitism  is  well  con- 
cealed. The  Bird's-nest  Orchis  furnishes  an 
excellent  illustration  of  this. 

The  bird's-nest  orchis  (Neottia,  Fig.  21)  is  a 
fairly  common,  though  frequently  overlooked 
inhabitant  of  the  humus  soil  of  dense  wood- 
lands. It  lives  under  the  ground  in  the  leaf 
mould,  except  when  it  pushes  up  its  cluster 
of  sickly  looking  flowers  on  a  yellowish  or 
brown  stem  in  early  summer.  No  green 
leaves  are  produced,  though  the  flowering 
shaft  bears  rather  large  brown  ones.  Hence 
the  plant  is  unable  to  manufacture  carbo- 
hydrate food  for  itself  in  the  way  that  its 
green  relatives  can  do.  Traced  below  the 
soil,  the  flowering  stalk  is  seen  to  spring  from 
a  short,  stumpy  root-stock  from  which  arises 
a  huddled  crowd  of  short,  brittle  roots. 

The  special  interest  of  the  bird's-nest 
orchis  in  the  present  connection  centres  in 
these  roots,  for  it  is  through  them  that, 
somehow  or  other,  the  stores  of  food  locked 
up  in  the  humus  soil  are  absorbed  by  the 


Fig.  21. — Neottia  Nidus-avis,  a  saprophytic  orchid. 
191 


192  PLANT  LIFE 

plant.  Microscopic  examination  of  a  root 
shows  that  they  are  permeated  by  fungal 
hyphce,  and  careful  experiments  have  proved 
that  it  is  through  the  intermediation  of  these 
fungal  threads  that  the  saprophyte  chiefly 
obtains  its  food.  It  thus  appears  that  the 
term  saprophyte  is  not  a  very  happy  one 
as  applied  to  a  plant  like  Neottia.  The 
relation  is  rather  more  akin  to  parasitism, 
and  it  is  the  fungus  from  which  nourishment 
is  finally  extorted.  But  inasmuch  as  the 
root  both  houses  the  fungus,  and  also  con- 
tributes something  towards  its  support,  the 
parasitism  is  not  very  one-sided,  although 
the  final  balance  lies  with  the  flowering  plant. 

This  association  of  the  root  with  a  fungus 
is  a  very  intimate  one  in  a  large  number 
of  instances,  and  it  occurs  in  a  very  great 
number  of  plants  which  would  never  be  sus- 
pected of  parasitic  habits.  The  fungus-root 
is  often  called  a  mycorhiza,  and  it  is  worth 
while  to  study  it  a  little  more  closely. 

The  roots  of  many  of  our  forest  trees 
produce  few  or  no  root-hairs.  Instead  of 
this  they  are  closely  invested  with  a  hairy 
coating  of  fungal  hyphge.  Not  only  do  these 
hyphae  ramify  in  the  soil,  but  they  also  enter 
the  root  itself.  Sometimes,  as  in  the  pines, 
they  only  pass  between  the  cells,  and  do  not 
enter  them,  but  in  other  cases,  as  for  example 
in  orchids  generally,  they  pierce  the  cell  walls 
and  enter  the  living  cells.  In  both  of  these 
types  of  mycorhiza  the  fungus  is  doubtless 


COMPOUND  ORGANISMS         193 

attracted  to  the  root  by  substances  which 
have  a  food  value  for  its  hyphse,  just  as  para- 
sitic fungi  are  induced  to  enter  the  bodies 
of  their  victims.  But  in  a  mycorhizal 
association  the  cells  of  the  root  control  the 
degree  of  invasiveness  which  the  fungus  can 
manifest,  and  not  only  so,  but  they  often 
proceed  to  actually  digest  the  fungus  itself 
after  it  has  flourished  within  them,  and  at 
their  expense  for  a  while. 

We  have  here,  then,  a  beautiful  example 
of  two-sided  parasitism,  in  which  the  final 
balance  of  profit  very  clearly  lies  with  the 
flowering  plant.  It  is  practically  certain  that 
the  fungus  obtains  some  carbohydrate  food, 
at  first  at  any  rate,  but  in  return  for  this  the 
plant  acquires  mineral  substances  in  solution, 
which  the  fungus  absorbs  from  the  soil. 
A  considerable  number  of  flowering  plants 
are  unable  to  thrive  unless  their  roots  become 
infected  in  this  way.  This  is  especially  true 
of  orchids.  Indeed,  one  of  the  great  diffi- 
culties experienced  in  raising  these  plants 
from  seed  has  been  solved  by  supplying  the 
young  seedling,  during  its  germination,  with 
the  fungus  appropriate  to  it.  And  so  close 
has  the  degree  of  association  between  fungus 
and  flowering  plant  become,  both  in  orchids 
and  in  many  other  plants,  that  neither 
can  grow  properly  in  the  absence  of  the 
other. 

Now  this  intimate  mycorhizal  relationship 
is  found  to  exist  in  all  the  flowering  sapro- 

N 


194  PLANT  LIFE 

phytes,  and  it  is  reasonable  to  conclude  that 
the  loss  of  the  green  leaves  and  all  the 
structural  change  consequent  on  their  abor- 
tion is  directly  connected  with  the  growing 
ability  of  the  saprophytic  plants  to  develop 
the  physiological  faculty  of  utilising  the 
resources  thus  rendered  available.  They 
come  more  and  more  to  depend  exclusively 
on  the  nutritive  processes  of  fungi,  not  only 
for  their  carbohydrate  kind  of  food  but  for 
the  still  more  complex  nitrogenous  nutriment 
as  well.  Of  course,  the  decaying  leaves  and 
other  vegetable  matter  in  the  soil  maintain 
a  plentiful  and  practically  continuous  supply 
of  carbonaceous  food  which  is  constantly  at 
the  disposal  of  those  organisms  which  are 
adapted  to  make  use  of  it.  There  is  little 
doubt  also  that  during  the  decomposition 
of  the  carbonaceous  humus  free  nitrogen  is 
sometimes  forced  into  combination  with  other 
elements,  either  by  micro-organisms,  or  myco- 
rhiza,  or  both,  and  so  is  rendered  available 
for  the  higher  plants. 

This  way  of  looking  at  the  matter  fits  in 
with  the  very  remarkable  nutritive  processes 
so  characteristic  of  the  leguminous  plants  of 
which  the  peas  and  clovers  are  representative 
examples.  If  one  of  these  plants  be  dug  up 
(Fig.  22),  its  roots  will  be  seen  to  bear  nodular, 
or  wart-like,  swellings.  These  swellings  are 
due  to  luxuriant  growth  of  the  tissues  of  the 
cortex  or  rind.  Examined  microscopically  the 
cells  are  found  to  contain  enormous  numbers  of 


COMPOUND  ORGANISMS         195 

bacteria-like  organisms  to  which  the  name  of 
Bacillus  radicola  has  been  given. 

The  root  becomes  infected  by  this  bacillus 


Fig.  22. — Root  tubercles  on  the  roots  of  the  Kidney- 
bean  plant. 

from  the  soil,  in  ordinary  samples  of  which 
it  is  apparently  always  present.  The  bacillus 
enters  through  a  root-hair,  and  when  it  reaches 
the  interior  of  the  cortex  it  multiplies  there, 


196  PLANT  LIFE 

producing  the  nodular  outgrowths  in  question. 
It  feeds  and  grows  mainly  at  the  expense  of 
the  sugars  and  other  substances  supplied  by 
the  host  plant,  these  having,  of  course,  been 
produced  as  the  result  of  the  photosynthetic 
activity  of  its  leaves. 

But  when  thus  provided  with  carbohydrate 
food,  the  bacillus  is  able  to  manufacture  the 
essential  nitrogenous  compounds  necessary  for 
the  production  of  protoplasm  by  utilising  the 
free  nitrogen  of  the  air.  Most  plants  have  to 
take  in  their  nitrogen  in  a  combined  form,  as 
ammonia  salts,  nitrates,  etc.,  for  nitrogen  is 
a  very  inert  element,  and  difficult  to  force  into 
combination  with  others.  Bacillus  radicola 
is  one  of  the  very  few  organisms  which  can 
perform  this  really  stupendous  task,  provided 
that  it  is  supplied  with  the  means  of  obtaining 
the  energy  required  for  the  process  in  the 
form  of  appropriate  carbohydrate  nutrition. 
There  is  no  doubt  as  to  the  facts,  for  the 
bacillus  will  do  the  same  thing  when  culti- 
vated outside  the  body  of  the  plant,  and 
under  the  most  rigidly  controlled  experimental 
conditions. 

After  the  bacilli  have  thriven  for  a  while, 
mainly  at  the  expense  of  the  food  supplied 
by  the  root  in  which  they  are  living  and  multi- 
plying, a  change  comes  over  them.  Many  of 
the  individuals  become  weaker,  and  undergo 
a  sort  of  degeneration,  whilst  a  few  pass  into 
a  resting  stage  in  which  they  become  highly 
resistant  to  adverse  conditions  of  life.  The 


COMPOUND  ORGANISMS         197 

leguminous  plant,  which  hitherto  has  been 
paying  out  carbohydrate  food  to  the  bacillus, 
now  begins  to  receive,  and  the  harvest  is 
a  rich  one,  for  it  acquires  from  the  degenerat- 
ing mass  of  bacilli  the  stores  of  nitrogenous 
matter  they  have  accumulated,  and  this  affords 
a  very  good  return  for  the  sugars,  etc.,  which 
it  had  previously  expended. 

The  comparatively  few  surviving  bacilli 
serve  to  infect  the  soil,  as  the  roots  gradually 
rot,  and  they  thus  are  enabled  to  attack  the 
roots  of  new  leguminous  plants  with  which 
they  may  be  brought  into  contact. 

In  comparing  the  leguminous  plants  with 
the  saprophytes  and  parasites  that  have 
undergone  simplification  (or  u  degeneration  ") 
of  vegetative  structure  we  can  readily  under- 
stand why  they  have  not  lost  their  green 
leaves,  and  all  that  the  possession  of  green 
leaves  entails.  For  a  continuous  supply  of 
carbohydrate  is  essential  for  the  growth  of 
the  bacilli,  and  without  it  there  is  no  manu- 
facture of  nitrogenous  substance  from  the 
free  nitrogen  of  the  air.  Moreover,  the  Legu- 
minosae  have  by  no  means  abandoned  the 
absorption  of  nitrates  from  the  soil.  The 
combined  nitrogen  they  acquire  from  the 
bacilli  is,  for  most  of  them  at  all  events, 
rather  of  the  nature  of  an  additional  supply, 
though  it  will,  and  often  does,  enable  them 
to  thrive  under  conditions  of  nitrogen  starva- 
tion which  would  be  fatal  to  the  majority 
of  other  plants.  Ultimately,  however,  they 


198  PLANT  LIFE 

owe  their  faculty  of  "  fixing  "  free  nitrogen 
to  the  energy  which  the  chlorophyll  enables 
them  to  obtain  from  the  sunlight.  This  is 
the  motive  power  which  enables  the  machinery 
of  the  green  leaf  to  maintain  its  output  of 
carbohydrate,  and  it  is  from  this  carbohydrate 
that  the  power  or  energy  is  more  immediately 
derived  which  enables  the  bacillus  to  perform 
the  tremendous  operation  of  forcing  free 
nitrogen  into  combination,  and  thus  to  build 
up  from  the  raw  materials  the  stuff  from  which 
protoplasm  itself  can  be  made.  Although 
the  leguminous  root  ultimately  profits  by 
its  relations  with  the  bacillus  in  thus  ac- 
quiring a  costly  food  in  exchange  for  a  cheap 
one,  there  is  no  indication  of  any  degeneration 
of  leaf  structure  on  the  part  of  the  flowering 
plant.  It  even  becomes  almost  unthinkable 
that  it  could  occur,  inasmuch  as  the  continuous 
supply  of  carbohydrate  from  the  green  parts 
is  a  prime  condition  of  the  nitrogenous 
synthesis.  The  importance  to  the  organic 
world  of  these  plants  which  bring  nitrogen 
into  combination  in  a  form  that  can  be 
utilised  by  living  beings  is  overwhelming. 
For  apart  from  some  means  of  maintaining 
the  supplies  of  nitrogenous  food,  life  itself 
would  ultimately  cease  to  be  possible  in  the 
world. 

There  are  many  other  instances  of  remark- 
able associations  of  two  or  more  plants,  in 
which  each  is  in  turn  more  or  less  parasitic 
on  the  other,  or,  at  the  least,  lives  on  the 


COMPOUND  ORGANISMS         199 

waste  products  formed  as  the  result  of  the 
chemical  life  processes  of  its  associate.  Such 
an  association  is  often  spoken  of  as  symbiosis, 
but  it  is  evident  that  the  transition  from 
symbiosis  to  parasitism  is  only  a  matter  of 
degree.  An  excellent  example  of  symbiosis  is 
furnished  by  Lichens.  These  plants  are  com- 
pound organisms,  made  up  of  a  fungus  on 
the  one  hand,  and  a  green  alga  on  the  other. 
It  is  often  possible  to  separate  the  two,  and 
to  cultivate  them  apart,  and  the  habit  of 
growth  (except  in  the  most  primitive  forms) 
is  very  different  from  that  which  occurs 
when  they  associate  to  form  the  lichen. 
Lichens  are  formed  in  countless  numbers 
every  spring,  and  scrapings  from  the  bark  of 
damp  trees  at  this  season  will  generally  yield 
quite  a  large  selection  of  these  compounded 
organisms  in  the  making.  Sometimes  a 
particular  fungus  filament  which  comes  in 
contact  with  an  appropriate  alga  may  be  seen 
to  branch  and  then  to  embrace  the  alga 
within  its  threads.  Many  of  these  early 
beginnings  of  lichens  are  really  due  to  the 
escape,  from  older  lichens,  of  algal  cells,  each 
of  which  is  already  accompanied  by  a  few 
fungal  hyphse.  These  young  associations — 
called  soredia — may  be  recognised  as  the  green 
or  grey  powdery  dust  which  often  occurs  on 
lichens  when  in  vigorous  growth. 

It  is  possible  to  make  a  lichen  artificially, 
by  bringing  together  the  alga  and  fungus. 
And  we  learn  that  one  fungus  may  attach 


200  PLANT  LIFE 

itself  to  several  kinds  of  algae,  and  vice  versa. 
In  every  instance,  however,  a  specific  lichen 
results  from  the  union  of  a  definite  fungus  with 
a  particular  alga — if  either  the  alga  or  fun- 
gus be  changed,  a  correspondingly  different 
"  species  "  of  lichen  is  formed. 

Both  organisms  thrive.  The  algal  cells  often 
become  unusually  large,  and  the  fungal 
mycelium  is  evidently  well  nourished.  But 
multiplication  of  cells  and  consequent  growth 
is  often  greatly  modified,  especially  in  the 
more  specialised  lichens  in  which  the  two 
organisms  become  more  intimately  dependent 
on  each  other. 

The  symbiosis  only  continues  to  pay  as 
long  as  the  alga  is  properly  exposed  to  light, 
and  for  so  long  as  it  is  properly  supplied 
with  water,  together  with  the  small  amount  of 
mineral  food  it  requires.  The  latter  offices 
are  largely  discharged  by  the  fungus,  which 
usually  attaches  the  lichen  to  the  substratum, 
whilst  its  gelatinous  walls  retain  the  water 
supplies  derived  from  intermittent  showers 
or  other  sources.  Thus  a  remarkable  degree 
of  correlation  is  displayed  in  the  growth 
processes  of  the  specialised  lichens,  and  some 
of  them  simulate  to  a  wonderful  extent  the 
form,  and  partly  even  the  structural  arrange- 
ments, to  be  met  with  in  the  green  leafy 
shoots  of  higher  plants  (Figs.  23A  and  B). 

Lichens  are  particularly  instructive  in 
showing  that  the  form  assumed  by  an  organ- 
ism is  in  the  long  run  determined  by  the 


COMPOUND    ORGANISMS 


201 


chemical  reactions  that  have  gone  on  and 
are  still  going  on  within  it.  These  reactions 
are  nicely  adjusted,  and  are  readily  interfered 
with  or  encouraged  by  the  conditions  under 
which  they  take  place.  The  result  is  per- 
ceived in  a  delicate  adjustment  of  growth 


Fig.  23A. — Pdtigera  canina,  a  lichen.     P,  the  fructification. 

whereby  the  different  parts  are  so  correlated 
to  each  other  that  excessive  development 
of  one  part  carries  with  it  its  own  order  of 
arrest,  whilst  deflection  of  nutrition  to  or 
from  any  part  will,  of  course,  correspondingly 
affect  growth  in  that  region. 

But,  nevertheless,  the  parts  of  which  the 


202 


PLANT  LIFE 


compound  organism  is  made  up  largely  deter- 
mine the  broad  lines  of  possible  development. 
If  we  alter  the  species,  whether  of  alga  or  of 


Fig.  23s. — Transverse  section  through  the  thallus  or  frond  of 
a  lichen  (Peltigera  canina).     A,  algal  cells. 

fungus,  we  have  seen  that  we  correspondingly 
affect  the  "  species  "  of  lichen,  ,as  determined 
and  denned   by   its    own  peculiar  form  and 
properties — i.  e.  its  process  of  development. 
Qf    course,  the    combinations    which    are 


COMPOUND  ORGANISMS          203 

encountered  in  nature  are  just  those  which  are 
fitted  for  actual  environmental  conditions. 
Plants  not  so  adapted  are  unable  permanently 
to  occupy  any  position  at  all.  The  positively 
unfit  are  speedily  exterminated,  and  only 
those  combinations  which  give  good  results 
can  persist.  But  the  "  good  results "  are 
primarily  the  result  of  particular  operation 
of  internal  factors.  They  arise  as  the  in- 
evitable consequence  of  the  particular  algal 
and  fungal  combination,  and  they  are  quite 
independent  and  irrespective  of  ultimate 
adaptation  to  light  or  other  external  conditions. 
These  external  conditions  are  the  tests  which 
largely  determine  not  the  origin,  but  the 
persistence  (or  extinction)  of  each  and  every 
individual  sort  of  lichen. 

It  may  not  be  possible  to  push  very  far 
our  analysis  of  the  factors  involved  in  the 
genesis  of  form  and  structure  on  the  one  hand, 
and  those  correlations  of  growth  wherein  so 
many  "  adaptive  modifications  "  consists  on 
the  other.  It  may  well  be,  however,  that 
an  experimental  study  of  lichens  is  destined 
to  throw  light  on  much  that  is  now  obscure. 
For  in  witnessing  the  synthesis  of  a  lichen, 
and  the  modification  in  structure  and  habit 
which  results  from  the  association  of  the  two 
symbionts,  we  seem  to  have  caught  a  glimpse 
of  the  secret  methods  and  processes  which 
direct  the  evolution  of  organic  form. 


204  PLANT  LIFE 


CHAPTER  XVIII 

VEGETATIVE   REPRODUCTION 

REPRODUCTION,  in  its  simplest  and  most 
primitive  form,  is  one  of  the  most  obvious 
results  of  growth.  It  represents,  after  a 
fashion,  and  in  a  certain  tangible  form,  the 
balance  of  profit  over  expenditure  on  the 
part  of  the  individual,  which  is  applied  to 
the  extension  of  the  business  of  the  species 
or  race.  But  the  process  is  not  a  simple  one. 
When  a  unicellular  plant,  Chlamydomonas 
for  example,  has  reached  a  certain  size,  the 
protoplasm  ceases  to  grow.  It  divides,  and 
the  products  of  this  fission,  which  may  be 
repeated  several  times,  separate  from  each 
other  as  new  and  independent  individuals. 
Nothing  is  left  of  the  old  organism,  it  has 
simply  split  up  into  a  number  of  smaller  ones, 
In  other  words,  the  nutritional  processes  which 
enabled  growth  to  proceed  have  prepared 
the  way  for,  and  have  then  given  way  to,  a 
new  set  of  chemical  processes,  and  these 
result  in  the  cleavage  of  the  mass  into  smaller 
parts.  This  cleavage,  or  cell-multiplication, 
may  be  started  in  several  different  ways,  but 
the  method  most  often  encountered  in  nature 
clearly  depends  on  factors  which  are  them- 


VEGETATIVE  REPRODUCTION    205 

selves  more  or  less  intimately  connected  with 
an  abundant  supply  of  nutrition. 

The  rapidity  with  which  many  of  these 
simple  plants  can  multiply,  provided  the 
nutrition  conditions  are  favourable,  is  truly 
astonishing.  Instances  are  not  uncommon, 
especially  among  bacteria,  in  which  a  cell 
colony  will  double  its  numbers  every  twenty 
minutes  or  so.  That  is  to  say,  in  about 
twelve  hours  one  cell  might  give  rise  to  nearly 
seventy  thousand  million  cells.  It  is  highly 
improbable  that  anything  approaching  this 
number  would  actually  be  reached,  because 
as  the  colony  begins  to  grow  the  individuals 
composing  it  compete  with  each  other  for  the 
food  supply,  and  those  more  centrally  situated 
will  obviously  be  at  a  disadvantage  in  this 
respect.  Many  other  conditions,  also  de- 
pending on  the  crowding  of  the  cells,  will 
begin  to  make  their  effects  felt  on  the  repro- 
ductive capacity  of  the  members  of  different 
portions  of  the  colony. 

Now  what  is  true  of  a  colony  of  detached 
individuals  is  still  more  applicable  as  soon  as 
the  dividing  cells  cease  to  separate  at  once 
from  each  other.  This  naturally  follows  from 
the  simple  geometrical  fact  that  if  the  cells 
are  all  growing  and  dividing  equally  and  in 
all  directions,  the  surface  of  the  cell  colony 
only  increases  as  the  square  of  the  radius  of 
the  growing  spherical  mass,  whilst  its  mass 
increases  as  the  cube.  The  difference  in 
available  nutrition  evidently  must  affect  the 


206  PLANT  LIFE 

growth  of  individual  cells,  and  hence  the 
shape  of  the  colony  as  a  whole.  Doubtless 
the  elongated,  narrow  cylindrical  form  of 
fungal  hyphae  is  to  be  interpreted,  in  part 
at  any  rate,  as  an  expression  of  this  fact. 
Vegetative  reproduction  tends,  in  such  forms, 
to  occur  by  the  transverse  fission  of  the 
cylindrical  growths ;  but,  as  we  have  already 
seen,  multiplicative  processes  are  not  identical 
with  those  of  growth,  and  both  in  the  fungi 
and  in  other  lowly  plants,  nutrition  sets  other 
processes  in  action  which  lead  to  the  formation 
of  various  sorts  of  specialised  reproductive 
cells.  This  does  not,  however,  interfere  with 
the  ordinary  multiplication  by  fission,  which 
still  remains  as  a  common  feature  among  them. 
In  the  evolution  of  the  more  complex 
plants,  the  cells — the  primitive  individuals — 
become  organised  into  a  higher  individuality. 
The  sense  in  which  we  use  the  term  repro- 
duction gradually  and  insensibly  changes, 
and  we  distinguish  between  cellular  multipli- 
cation and  the  reproduction  of  the  multicellular 
individual.  We  may  still  think  of  the  multi- 
plication of  cells  as  reproduction  in  the 
abstract,  but  our  unit  organism,  so  to  speak, 
has  become  transformed;  it  is  no  longer 
identical  with  the  isolated  cell,  but  is  repre- 
sented by  the  cell  colony.  Reproduction  in 
such  a  colony,  concretely  considered,  comes 
then  to  signify  the  process  by  which,  not  new 
cells  only,  but  new  colonies  are  started.  It  is 
a  change  in  the  point  of  view. 


VEGETATIVE  REPRODUCTION     207 

The  most  common  method  by  which  the 
simpler  aquatic  algae  reproduce  themselves 
vegetatively  is  by  giving  birth  to  zoospores. 
The  protoplasmic  contents  of  a  cell  contract 
away  from  the  wall,  cilia  are  developed,  and 
the  zoospore  escapes  through  a  hole  which  is 
formed  in  the  cell  wall.  Very  often  a  series 
of  adjacent  cells  may  be  seen  all  to  give  rise 
to  zoospores  in  this  way.  Sometimes  the 
zoospores  are  not  so  simple,  and  represent  not 
single  cells  only  but  a  cluster,  the  individuals 
of  which  are  not  delimited  by  walls  from 
each  other.  The  huge  zoospore  of  Vaucheria 
belongs  to  this  type ;  it  is  easily  visible  to  the 
naked  eye,  as  it  rolls  about  through  the  water 
by  means  of  its  numerous  pairs  of  cilia. 

But  however  the  zoospores  are  formed, 
they  generally  settle  down  after  a  period 
of  independent  movement.  They  withdraw 
their  cilia,  secrete  a  cell  wall  over  their  naked 
surface,  and  grow  into  an  organism  generally 
similar  to  that  from  which  they  themselves 
have  sprung. 

It  is  different  with  land  plants.  Motile 
propagative  bodies  would  be  practically 
useless  here,  and  the  nakedness  of  the  proto- 
plasm would  render  them  specially  susceptible 
to  numerous  adverse  influences  inseparable 
from  existence  on  land.  In  the  simpler  forms 
we  find  that  entire  cells,  i.  e.  protoplasts 
which  remain  enclosed  in  cellulose  membranes, 
replace  the  naked  zoospore.  From  this  simple 
stage  the  rest  is  easy.  A  few  coherent  cells 


208  PLANT  LIFE 

become  detached  as  a  sort  of  bud  or  gemma, 
and  so  reproduce  the  parent  plant.  Mosses 
and  liverworts  are  freely  reproduced  in  this 
manner.  The  gemmse  are  of  all  shapes  and 
sizes.  They  may  be  produced  in  a  variety 
of  ways,  e.  g.  as  biscuit-like  outgrowths  from 
the  leaves,  and  sometimes,  as  in  the  moss 
Tetraphis,  the  whole  of  the  leaves  at  the 
growing  point  of  older  stems  may  develop 
into  reproductive  bodies  of  this  kind.  Pro- 
pagative  outgrowths  may  also  occur  on  the 
underground  parts  of  the  stems  of  mosses  and 
liverworts,  and  they  are  often  filled  with 
reserves  of  food.  Thus  they  enable  the 
species  to  tide  over  periods  of  drought,  etc., 
which  might  easily  prove  fatal  to  the  indi- 
vidual. On  the  return  of  better  conditions 
they  sprout,  and  thus  reproduce  the  plant 
afresh. 

Passing  to  the  higher  plants,  the  vege- 
tative propagative  processes  are  seen  to 
exhibit  almost  infinite  variety.  The  smallest 
parts  of  some  plants  are  capable  of  reproducing 
the  whole — as  any  one  may  discover  who 
endeavours  to  eradicate  troublesome  weeds, 
e.  g.  bindweed,  from  a  garden.  The  regular 
storage  organs,  bulbs,  tubers,  etc.,  are  speci- 
ally fitted  to  serve  as  propagative  organs  on 
account  of  the  stock  of  organic  food  they 
contain.  Bulbs,  for  example,  consisting  of 
a  short  squat  stem  bearing  fleshy  leaves, 
form  the  ordinary  propagative  bodies  of  lilies. 
Even  a  single  scale,  detached  from  the  bulb 


VEGETATIVE  REPRODUCTION    209 

and  planted  in  soil,  will  commonly  give  rise 
to  new  plants,  and  this  faculty  is  taken 
advantage  of  in  propagating  new  and  valuable 
species. 

Sometimes  young  plantlets  are  produced 
by  the  development  of  a  cluster  of  cells  which 
still  remains  attached  to  the  parent  plant. 
This  happens  in  many  ferns,  where  bulbils 
are  formed  on  the  leaves  or  leaf  stalks,  and 
when  they  are  set  free  they  are  already 
provided  with  all  the  organs  necessary  to 
start  at  once  into  growth.  The  process  of 
propagation  by  gemmae  and  by  young  plantlets 
is  essentially  the  same,  the  difference  consists 
in  the  particular  stage  of  development  which 
is  reached  when  the  propagative  body  is  cast 
adrift  from  the  parent.  The  gemma  is  shed 
at  an  early  stage,  while  the  bulbil  represents 
a  gemma  that  has  remained  to  develop  on  the 
parent  plant,  and  has  been  fed  at  its  expense 
during  the  early  stages  of  growth.  But  there 
are  advantages  and  disadvantages  in  both 
methods.  The  gemmae  are  small,  and  are 
more  readily  dispersed  over  wide  distances 
than  the  larger  young  plants.  Furthermore, 
the  latter  by  their  very  complexity  are  more 
liable  to  perish  unless  they  speedily  reach  a 
spot  in  which  they  find  conditions  suitable 
for  immediate  development. 

But  in  spite  of  these  numerous  and  elaborate 
kinds  of  vegetative  reproduction,  most  plants 
still  retain  the  primitive  capacity  of  merely 
regenerating  lost  parts  to  a  surprising  extent, 


210  PLANT  LIFE 

a  circumstance  of  which  advantage  is  taken 
in  the  propagation  of  valued  species  and 
varieties.  Everybody  knows  how  simple  it 
often  is  to  increase  a  plant  by  cuttings. 
Sometimes  cuttings  of  roots  will  grow  just 
as  easily  as  those  of  stems,  and  even  the 
leaves  of  some  plants  may  be  used  with  almost 
certain  chances  of  success.  Begonias,  for 
example,  and  certain  other  greenhouse  plants, 
are  generally  propagated  in  this  way. 

Again,  in  the  operations  of  budding  and 
grafting,  we  see  how  the. process  of  cell  division 
and  multiplication  is  followed  by  cohesion; 
the  bud  or  the  graft  "takes,"  becomes 
united  with  the  tissues  of  the  stock.  Instead 
of  the  bud  or  cutting  being  planted  in  the 
soil,  it  is  here  planted  on  to  another  organism. 
And,  in  passing,  we  may  note  that  the  graft 
produces  no  roots,  as  it  would  have  done  if 
planted  in  the  soil.  The  internal  stimulus 
which  might  have  led  to  root  production  is 
absent,  inhibited,  perhaps,  by  the  nutrition 
that  is  plentifully  poured  in  from  the  tissues 
of  the  plant  on  which  the  bud  or  graft  is 
growing. 

All  the  various  examples  of  multiplication 
and  propagation  to  which  allusion  has  been 
made  in  this  chapter  are  instances  of  what 
may  best  be  called  vegetative  reproduction  or 
propagation,  and  they  are  seen  to  be  intimately 
related  with  the  functions  of  growth  and 
nutrition.  They  represent  various  methods 
of  dividing  up  the  individual,  and  the  liberated 


VEGETATIVE  REPRODUCTION    211 

portions  grow  up  into  plants  like  those  from 
which  they  have  themselves  been  derived. 
From  simple  beginnings  the  propagative 
bodies  advance  in  complexity,  and  other 
structures,  not  in  the  first  instance  differenti- 
ated as  propagative  bodies  (e.  g.  thickened 
stems  in  which  food  is  stored),  easily  assume 
this  function  of  vegetative  reproduction. 
One  may  often  trace  the  stages  by  which 
this  is  brought  about  within  the  limits  of  a 
group  of  closely  related  species.  The  Jerusa- 
lem Artichoke,  a  sort  of  sunflower,  is  connected 
by  all  imaginable  transitions  with  other 
species  in  which  the  underground  stems  have 
not  yet  proceeded  to  form  tubers  (as  in  the 
artichoke),  but  exist  as  mere  whip-like  runners 
which  turn  up  and  only  grow  to  new  plants 
by  the  slow  and  accidental  process  of  rotting 
off  their  connection  with  the  parent  plant. 
In  others  the  propagative  character  is  still 
less  evident,  and  the  storage  function  is 
absent  altogether.  Finally,  there  are  many 
sunflowers  which  normally  fail  to  produce 
any  underground  runners  at  all. 

Thus,  in  spite  of  the  endless  variety  in  the 
carrying  out  of  the  process,  the  essential 
character  of  vegetative  propagation  is  really 
a  simple  one.  In  this  respect  it  stands  in 
marked  contrast  to  the  other,  the  sexual, 
reproductive  process,  which  will  form  the 
subject  of  the  next  chapter. 


212  PLANT  LIFE 


CHAPTER  XIX 

SEXUAL    REPRODUCTION 

SEXUAL  reproduction  occurs  in  almost  all 
the  divisions  of  the  animal  and  vegetable 
kingdoms,  although  it  has  not  as  yet  been 
detected  in  some  of  the  lower  groups.  These 
consist  either  of  organisms  of  extreme  sim- 
plicity, or  of  those  in  which  we  have  grounds 
for  believing  that  sexuality  has  been  lost, 
probably  in  connection  with  special  conditions 
of  nutrition.  In  some  of  the  higher  plants 
the  sexual  function  has  degenerated,  though 
we  cannot  clearly  trace  the  loss  to  any  definite 
cause. 

The  most  striking  peculiarity  connected 
with  sexual  reproduction,  next  to  its  almost 
universal  occurrence,  lies  in  its  remarkably 
complex  character.  Moreover,  its  effects  on 
the  development  of  the  vegetable  kingdom 
have  been  extremely  far-reaching,  and  have 
profoundly  influenced  the  direction  of  evolu- 
tionary progress,  as  interpreted  by  a  study 
of  the  life-history  of  the  plants  themselves. 

The  sexual  act  itself  stands  in  strong 
antithesis  to  vegetative  propagation,  for  it 
does  not  directly  involve  an  increase,  but  a 
reduction  in  the  number  of  cells.  Two  cells, 


SEXUAL  REPRODUCTION       213 

which  we  may  call  the  gametes,  are  concerned 
in  the  process,  and  they  invariably  coalesce 
to  form  one — the  zyg&te. 

From  the  zygote,  which  is  always  a  single 
cell,  there  springs  a  new  generation  which 
may  multiply  in  various  ways,  but  sooner 
or  later  a  process  supervenes  which  leads 
once  more  to  the  formation  of  new  gametes. 
These  in  their  turn  may  coalesce  in  appropriate 
pairs  and  so  form  new  zygotes. 

In  the  more  primitive  unicellular  plants  the 
sexual  cells  or  gametes  are  often  apparently 
precisely  similar  to  each  other.  They  may 
also  be  externally  indistinguishable  from  the 
ordinary  vegetative  organism  itself,  or  at 
any  rate  from  the  newly  formed  individuals 
which  have  just  arisen  by  vegetative  propaga- 
tion. Nevertheless  the  sexual  individuals  are 
physiologically  very  distinct.  If  it  were  not 
so,  they  would  scarcely  be  definitely  impelled 
to  unite,  and  to  unite  only  in  pairs. 

Closer  examination  reveals  the  fact  that 
in  sexual  union  the  coalescence  of  the  gametes 
is  a  very  intimate  one.  Not  only  do  the 
extra-nuclear  protoplasms  flow  together,  but 
the  two  nuclei  also  unite  and  mingle  their 
contents  in  common.  A  study  of  the  higher 
types,  both  of  animals  and  plants,  leads  to 
the  further  conclusion  that  it  is  in  the  nuclear 
fusion,  more  than  in  anything  else,  that  the 
significance  of  the  sexual  act  is  to  be  sought. 
We  shall  return  to  this  point  later,  but  it  will 
be  convenient  and  profitable  in  the  first 


214  PLANT  LIFE 

place  to  glance  at  a  few  examples,  in  order  to 
gain  some  knowledge  of  the  general  character 
of  the  sexual  process  itself  so  far  as  we  at 
present  understand  it.  At  the  same  time,  we 
shall  be  in  a  better  position  to  appreciate  the 
bearings  of  its  elaboration  on  the  evolution 
of  the  series  of  higher  plants. 

If  we  once  more  take  as  our  starting-point 
a  relatively  simple  unicellular  plant  such  as 
Chlamydomonas,  we  find  that  under  certain 
conditions  it  continues  to  grow  and  to  multiply 
itself  vegetatively  (see  p.  15).  After  a  time, 
however,  and  under  certain  altered  nutritive 
conditions,  sexual  reproduction  sets  in  (Fig. 
24).  The  young  individuals  which  have  been 
recently  liberated  from  parent  cells,  after 
swimming  about  for  a  while,  undergo  a  change. 
The  living  protoplasmic  body  slips  out  of 
the  cellulose  skin,  and  swims  as  a  naked  cell 
in  the  water.  Very  soon  these  cells  are 
observed  to  approach  one  another  in  pairs, 
Two  individuals  become  attached,  and  then 
gradually  coalesce.  The  cilia  disappear,  and 
the  now  motionless  zygote  becomes  spherical 
and  surrounds  itself  with  a  new  cell  wall. 
Chemical  changes  continue  to  go  on  within 
its  body,  for  the  chlorophyll  loses  its  green 
colour  and  gives  way  to  a  red  pigment. 
Later  on,  and  after  a  longer  or  shorter  period 
of  rest,  the  green  colour  returns,  the  cell 
reawakens  to  vegetative  activity,  its  contents 
divide,  and  new  chlamydomonas  individuals 
are  produced. 


SEXUAL  REPRODUCTION       215 

Now  the  chlamydomonas  is  an  especially  in- 
teresting organism  inasmuch  as  it  responds 


Fig.  24. — CMamydomonas  media,  sexual  process.  I. — Sexual 
cell  (gamete).  II. — The  same,  somewhat  more  advanced. 
III. — The  protoplasts,  which  have  escaped  from  their 
cellulose  membranes,  just  in  contact.  IV. — The  fusion 
nearly  complete,  note  the  two  nuclei  (dark  spots),  one  in 
each  gamete.  V. — Complete  protoplasmic  fusion  of  two 
gametes,  but  the  two  nuclei  still  distinct.  VI. — The  zygote 
after  complete  fusion,  surrounded  by  a  new  membrane. 

with  great  precision    to  various  changes  in 
external  conditions  which  are  able  to  influence 


216  PLANT  LIFE 

its  nutrition.  It  is  possible  to  maintain  the 
plant,  apparently  for  an  indefinite  period,  in 
a  state  of  vegetatively  active  growth.  On 
the  other  hand,  it  may  with  almost  equal 
certainty  be  compelled  to  enter  on  the 
sexually  reproductive  phase  of  its  life.  A 
sudden  starvation,  if  previously  well  nourished, 
and  so  long  as  the  organisms  are  exposed  to 
light,  will  at  once  bring  about  the  change 
that  leads  to  the  formation  of  gametes.  But 
we  may  at  once  confess  that  we  do  not  as 
yet  understand  how  these  conditions  work 
in  producing  the  observed  effects.  Nor  are 
we  able  to  form  a  clear  idea  as  to  why  the 
addition  of  nutritive  salts  to  the  water  in 
which  the  chlamydomonas  is  living  suffices 
at  once  to  arrest  sexual  development,  and  to 
switch  the  life  processes  back  on  to  the 
vegetative  course;  so  much  so,  indeed,  that 
even  gametes  can  develop  independently, 
and  in  a  vegetative  manner,  *.  e.  without  any 
sexual  union. 

But  the  effects  of  sudden  starvation  on 
previously  well-nourished  organisms  are  well 
known  to  conduce  to  the  development  of 
sexual  reproductive  organs.  In  a  chlamydo- 
monas the  organism  and  the  sexual  cell  are 
practically  identical,  and  it  is  in  the  highest 
degree  suggestive  to  find  that  what  stimulates 
the  production  of  sexual  organs  in  a  complex 
and  highly  differentiated  plant  will  also  cause 
the  undifferentiated  primitive  one  also  to 
enter  on  a  sexual  condition  or  phase.  More- 


SEXUAL  REPRODUCTION       217 

over,  the  converse  is  also  true,  though  it  is 
often  less  easily  demonstrated.  For  a  reversal 
of  the  conditions  that  led  to  the  development 
of  the  sexual  state  will  arrest  it,  and  cause  not 
only  lowly,  but  many  of  the  higher  plants 
to  resume  their  vegetative  growth.  Some  of 
the  malformations  often  seen  in  flowering 
plants,  as  the  consequence  of  injudicious 
manuring,  represent  the  results  of  the 
antagonism  between  the  sexual  and  vegetative 
functions. 

But  in  the  more  specialised  plants,  where 
the  sexual  and  other  reproductive  cells  are 
different  from  the  general  mass  of  the  body 
cells,  the  sexual  elements  themselves  are 
more  limited  in  their  range  of  development. 
We  can,  in  favourable  instances,  so  influence 
the  plant  as  to  determine  whether  or  not  it 
shall  form  sexual  organs.  But  where  once 
the  sexual  cells  are  formed,  these  can  seldom  be 
induced  to  develop  further,  unless  they  unite 
in  appropriate  pairs.  For  some  reason  the 
chemical  processes  no  longer  run  in  the 
direction  of  growth  and  development.  They 
result  in  death  and  disintegration  unless  a 
sexual  fusion  occurs. 

We  do  not  as  yet  know  why  this  should 
be  so,  but  the  experimental  work  of  recent 
years  has  taught  us  that  by  suitably  altering 
the  conditions  of  chemical  action  within  the 
protoplasm  of  the  gamete,  and  especially  by 
appropriately  regulating  the  oxidative  pro- 
cesses, the  cell  will  again  be  able  to  resume 


218  PLANT  LIFE 

vegetative  activity.  Loeb  and  others  have 
shown  this  to  be  experimentally  possible 
with  eggs  of  various  animals ;  and  although  it 
has  not  yet  been  satisfactorily  demonstrated 
in  plants,  this  is  largely  owing  to  the  very 
small  size  of  the  egg,  and  to  its  ordinary 
inaccessibility  for  purposes  of  this  kind  of 
experiment.  There  is  no  doubt  that  the 
essential  processes  are  identical  in  animals 
and  plants,  and,  moreover,  we  are  aware  of 
instances  amongst  the  latter  in  which  eggs 
can  be  stimulated,  though  by  indirect  means, 
to  grow  and  develop  in  the  absence  of 
fertilisation. 

We  do  not  as  yet  at  all  understand — and 
yet  this  lies  very  near  to  the  root  of  the  whole 
matter — why  the  sexual  change  should  pro- 
duce two  kinds  of  states.  We  speak  of  these 
states  as  male  and  female  respectively  in  the 
higher  forms,  but  there  is  no  detectable 
difference  between  the  gametes  of  the  sim- 
plest organisms.  Why  there  should  be  this 
difference  of  state,  and  why  the  coalescence 
of  two  individuals  should  not  only  obliterate 
it,  but  give  special  vigour  to  the  resulting  cell 
we  are  not  as  yet  in  a  position  to  declare. 

As  we  pass  from  the  lower  to  the  higher 
ranks  of  the  vegetable  kingdom,  we  find 
that  the  primary  physiological  differences  by 
which  sex  is  first  differentiated  are  betrayed 
by  secondary  changes  which  enable  the  male 
to  be  distinguished  from  the  female  gamete. 
The  general  trend  of  the  distinction  is  un- 


SEXUAL  REPRODUCTION        219 

mistakable,  and  is  of  considerable  import- 
ance in  its  connection  with  the  sexual  act. 
The  chief  character  which  urges  itself  on 
our  notice  consists  in  the  relatively  large 
size  of  the  egg  or  female  gamete,  and  the 
small  size  of  the  other,  the  male  or  sperm. 

The  egg  not  only  becomes  large,  but  it 
loses  the  power  of  independent  motility. 
It  consists  of  a  bulky  mass  of  cytoplasm,  in 
which  nutritive  matter  is  often  present,  and 
it  also  contains  a  large  and  somewhat  watery- 
looking  nucleus. 

The  sperm,  on  the  other  hand,  is  small  and 
compact.  It  is  nearly  always  actively  motile, 
though  this  character  is  almost  or  entirely 
abandoned  in  certain  groups,  such  as  the 
highest  flowering  plants  in  which  this  has 
evidently  occurred  as  the  result  of  correlation 
with  other  secondary  changes  connected  with 
pollination,  which  render  motility  useless 
or  even  disadvantageous.  In  another  im- 
portant respect  the  sperm  also  differs  from 
the  egg,  inasmuch  as  it  tends  to  become 
composed  almost  entirely  of  the  cell  nucleus, 
the  cytoplasm  being  merely  represented  by 
the  cilia  and  a  thin  skin  which  sheaths  the 
nucleus  as  a  whole. 

One  of  the  results  secured  by  fertilisation 
has  already  been  pointed  out,  namely, 
the  vigorous  development  so  characteristic 
of  the  sexually  produced  organism.  But 
there  is  another  and  perhaps  hardly  less 
important  consequence,  namely,  that  the 


220  PLANT  LIFE 

zygote  combines  within  itself  the  slightly 
different  properties  borne  by  the  egg  and 
sperm,  in  so  far  as  they  are  of  different 
origin.  This  must  be  specially  true  when 
the  gametes  spring  from  different  parents, 
for  there  is  no  doubt  as  to  the  transference, 
by  means  of  the  gametes,  of  the  hereditary 
qualities  of  the  organisms  from  which  the 
gametes  have  sprung. 

Experience  teaches  us  that  the  egg  and 
sperm  contribute  equally  towards  the  char- 
acters of  the  plant  which  will  develop  from 
the  zygote.  The  reason  for  this  almost 
certainly  lies  in  the  preponderant  share  taken 
by  the  nucleus  in  determining  the  organisa- 
tion of  the  individual,  The  sperm  and  egg 
contain  about  equal  parts  of  the  essential 
constituents  of  the  nucleus,  and  this  explains 
the  circumstance  that  the  minute  sperm  is  as 
potent,  from  the  point  of  view  of  the  trans- 
mission of  hereditary  characters,  as  is  the 
bulky  egg. 

These  two  functions,  rejuvenescence  and  the 
combination  of  diverse  hereditary  characters, 
then,  are  the  most  obvious  results  achieved 
by  fertilisation.  Probably  the  first-named 
function,  rejuvenescence,  is  the  more  primi- 
tive, and  the  chemical  affinity  between  the 
egg  and  sperm  first  arose  and  was  maintained 
by  the  primitive  conditions  that  made  fertili- 
sation a  conditio  sine  qua  non  of  further 
development.  But  the  second  was  inevi- 
tably bound  up  with  it.  This  latter  circum- 


SEXUAL  REPRODUCTION       221 

stance  was  fraught  with  tremendous  conse- 
quences which  were  destined  to  influence  the 
course  of  evolution  of  the  entire  organic 
world,  of  animals  no  less  than  plants. 

One  of  the  most  singular  features  of  the 
sexual  act,  in  so  far  as  it  can  be  actually 
observed,  consists  in  the  attraction  which 
the  gametes  exercise  on  each  other.  It  is 
by  this  means  that  fertilisation  is  rendered 
possible,  and  is  definitely  secured. 

As  the  differentiation  of  the  male  and  female 
gametes  becomes  more  pronounced,  the  im- 
mobile egg  is  ardently  sought  by  the  motile 
sperms,  and  the  latter  are  evidently  stimulated 
by  something  which  emanates  from  the  egg. 
Even  when  the  sperms  are  not  themselves 
vigorously  motile,  they  are  often,  as  in  the 
case  of  the  flowering  plants,  conducted  to 
the  egg  in  an  analogous,  though  more  indirect, 
method  in  which  attraction  plays  a  part. 
For  the  pollen  tube,  in  which  the  male  gametes 
are  contained,  grows  into  the  cavity  of  the 
ovary,  and  thence  to  the  ovule  in  which  the 
egg  is  formed,  and  it  there  discharges  them  in 
such  a  way  as  to  render  fertilisation  almost 
inevitable. 

But  it  is  simpler  to  choose  a  less  specialised 
type  than  the  flowering  plant  in  order  to 
become  familiar  with  the  essential  facts  of 
fertilisation.  For  this  purpose  some  of  the 
brown  seaweeds  (Fucus)  afford  admirable 
material.  They  produce  large  quantities  of 
eggs  and  sperms  in  little  conceptacles  situated 


222  PLANT  LIFE 

near  the  tips  of  the  fronds.  The  eggs  and 
sperms  are  extruded  from  the  conceptacles 
into  the  sea-water,  and  the  sperms  are  soon 
observed  to  be  actively  swimming  in  all 
directions.  At  first  the  eggs  exercise  no 
influence  upon  them,  but  as  the  membranes,  in 
which  they  are  at  first  enveloped,  dissolve  in 
the  water,  the  sperms  are  seen  to  cluster 
around  the  eggs,  and  each  egg  becomes  the 
centre  of  a  crowd  of  male  gametes  which 
are  endeavouring  to  gain  entrance  into  its 
substance.  Presently  one  slips  through  the 
peripheral  limiting  pellicle  of  the  protoplasm 
and  gains  the  interior  of  the  egg.  It  passes 
rapidly  through  the  cytoplasm  and  becomes 
appressed  to  the  egg-nucleus.  In  a  few 
seconds  it  swells  up,  and  finally  the  two 
nuclei,  belonging  to  the  egg  and  sperm 
respectively,  coalesce,  and  fertilisation  is  thus 
achieved. 

Now  it  is  a  remarkable  fact  that  during 
fertilisation  only  one  sperm  is  required  to 
fertilise  the  relatively  large  egg.  This  is  true 
of  animals  as  well  as  plants.  Experiments 
have  clearly  proved  that  normally  only  one 
male  cell  can  enter  the  egg  at  all,  and  that  in 
any  case  only  one  male  nucleus  fuses  with  the 
egg  nucleus.  The  study  of  seaweeds  has 
furnished  a  clue  to  the  means  by  which  the 
entrance  into  the  egg  of  but  one  of  the  crowd 
of  struggling  sperms  is  effected.  It  has  also 
thrown  light  on  some  important  features  of 
fertilisation  itself. 


SEXUAL  REPRODUCTION        223 

Certain  seaweeds  (Halidrys)  have  very  large 


Fig.  25. — Fertilisation  of  Halidrys  siliquosa.  I. — Egg  when 
first  extruded  into  the  sea-water.  II. — Later  stage,  egg 
spherical.  III. — Egg  suddenly  enlarged,  just  before 
fertilisation.  IV.— Fertilisation.  The  sperm  (S)  has 
slipped  in  at  the  "  crinkled  "  spot  (R).  V. — After  ferti- 
lisation the  egg  becomes  crinkled  all  over  and  no  longer 
attractive,  but  poisonous  to  sperms.  VI. — The  fertilised 
egg  has  contracted  and  is  surrounded  by  a  newly  formed 
membrane. 

eggs,  and  if  they  are  kept  in  sea-water  as 
they  are  extruded  from  the  conceptacles,  and 


224  PLANT  LIFE 

are  watched  under  the  microscope  while  the 
sperms  are  swimming  about  them,  they  will 
be  seen,  one  by  one,  suddenly  to  change 
their  form — they  swell  up,  and  at  some  one 
spot  on  their  surface  they  become  "  prickly." 
This  prickliness  spreads  with  great  rapidity 
over  the  surface  of  the  egg.  The  onset  of 
this  curious  appearance  marks  the  entrance 
of  a  sperm  into  the  egg.  The  immediate  effect 
of  that  sperm  on  the  egg  protoplasm  is  to 
render  it  not  only  no  longer  attractive  to 
the  rest  of  the  sperms,  but  actually  poisonous 
to  them.  An  explanation  is  therefore  at  once 
furnished  as  to  how  the  entrance  of  more 
than  one  sperm  is  prevented.  The  change  is 
a  sudden  one,  resulting  from  the  interaction 
of  the  substance  of  the  egg  and  sperm — a 
circumstance  which  sufficiently  emphasises 
the  physiological  difference  existing  between 
them.  Under  unfavourable  conditions,  e.  g. 
badly  aerated  water,  or  by  the  addition  of 
certain  substances  to  the  water,  the  sudden- 
ness of  this  reaction  can  be  slowed  down,  and 
then  it  may  happen  that  more  than  one 
sperm  effects  an  entrance.  But  it  seems  to 
be  a  general  rule  that  if  more  than  one  of 
them  fuses  with  the  nucleus  of  the  egg, 
either  no  further  development  takes  place, 
or  monstrous  embryos  are  produced  which 
commonly  die  during  the  earlier  stages  of 
development. 

It  is  evident,  then,  that  the  act  of  sexual 
fusion  produces  striking  and  immediate  change 


SEXUAL  REPRODUCTION        225 

in  the  egg,  and  that  the  fusion  of  the  two 
nuclei  is  an  essential  part  of  the  whole 
process. 

The  result  of  fertilisation  is  invariably  to 
start  a  series  of  chemical  changes  in  the  egg. 
The  first  of  these  changes  commonly  results 
in  the  secretion  of  a  membrane  over  its  outer 
surface,  and  then  a  period  of  quiescence 
usually  intervenes  before  any  further  visible 
development  begins.  After  the  lapse  of  a 
certain  time,  which  may  vary  within  rather 
wide  limits,  the  fertilised  egg  commences  to 
"develop." 

The  lines  along  which  development  proceeds 
differ  greatly  in  different  groups  of  plants. 
In  the  simpler  ones,  such  as  Chlamydomonas, 
no  apparent  growth  takes  place,  but  the 
zygote  divides,  giving  rise  to  a  number  of 
separate  cells  which  escape  as  zoospores  from 
the  zygote  membrane,  and  finally  grow  into 
as  many  different  individuals.  A  somewhat 
similar  course  is  pursued  by  many  other  algae, 
but  in  some  of  them  the  production  of  motile 
zoospores  is  postponed  until  after  an  embryo, 
composed  of  a  larger  or  smaller  number  of  cells, 
has  been  formed. 

In  the  higher  plants,  from  the  mosses 
upwards,  the  zygote  gives  rise  to  a  plant 
quite  unlike  that  from  which  the  gametes 
were  produced.  This  plant  forms  repro- 
ductive bodies  known  as  spores,  and  when 
the  spores  in  their  turn  germinate,  they 
give  rise  to  another  very  dissimilar  cellular 


226  PLANT  LIFE 

structure,  the  prothallus,  on  which  the  gametes 
are  ultimately  produced. 

It  is,  however,  impossible  to  appreciate  the 
significance  of  all  this  without  some  prelimin- 
ary acquaintance  with  the  behaviour  of  the 
cell  nucleus,  and  its  relation  to  cell  division 
and  cell  organisation,  which  will  form  the 
subject  of  the  following  chapter. 


CHAPTER  XX 

THE   CELL-NUCLEUS   AND   FERTILISATION 

IN  order  to  be  in  a  position  to  grasp  the 
essential  facts  of  fertilisation,  and  their  far- 
reaching  consequences  on  the  organism  in 
general,  it  is  necessary,  as  stated  in  the  con- 
cluding paragraph  of  the  preceding  chapter, 
to  learn  something  of  the  structure  of  the 
nucleus.  Moreover,  a  study  of  the  nuclear 
processes  will  enable  us  to  apprehend  the 
meaning  of  some  of  the  most  constant  and 
singular  features  which,  in  the  form  of  alterna- 
tion of  generations,  are  of  such  widespread 
occurrence  in  the  vegetable  kingdom. 


CELL-NUCLEUS—FERTILISATION    227 

The  nucleus  is  perhaps  the  most  important 
organ  of  the  cell.  There  are  strong  grounds 
for  believing  that  it  is  largely  concerned  in  the 
determination  of  those  hereditary  qualities 
which  distinguish  one  species  from  another; 
and  we  are  also  well  aware  of  its  great  import- 
ance in  governing  the  chemical  changes  which 
proceed  within  the  protoplasm. 

The  nucleus  consists,  essentially,  of  a 
variety  of  substances,  more  or  less  gelatinous 
in  consistency.  These,  together  with  more 
fluid  constituents,  are  contained  within  a 
membrane,  and  are  thus  sharply  delimited 
from  the  surrounding  cytoplasm.  The  con- 
tents of  the  nucleus  are  not  homogeneous. 
One  or  more  spherical  bodies,  the  nucleoli, 
may  often  be  seen  inside  it.  These,  although 
often  very  prominent,  are  of  subordinate  im- 
portance, inasmuch  as  they  chiefly  represent 
reserves  of  material  to  be  drawn  on  at  periods 
when  the  nucleus  is  undergoing  division.  The 
more  solid  gelatinous  matrix  (linin)  contains 
the  most  important  nuclear  constituents.  -A 
more  or  less  finely  divided  substance  distri- 
buted in  the  gelatinous  matrix  often  gives  the 
nucleus  a  rather  granular  appearance.  Stains 
of  various  kinds  render  this  much  more 
evident,  and  the  stainable  particles  are  often 
known  as  chromatin. 

When  the  nucleus  is  about  to  divide,  strik- 
ing rearrangements  are  observed  to  take  place 
within  it.  The  gelatinous  linin,  in  which  the 
chromatin  is  diffused,  contracts,  and  at  the 


228  PLANT  LIFE 

same  time  the  chromatin  increases  in  quan- 
tity. Stains  of  various  kinds  show  that  the 
chromatin -containing  strands  are,  as  it 
were,  becoming  individualised  within  the  nu- 
cleus, although  anastomoses  between  adjacent 
strands  are  still  of  common  occurrence.  As 
the  strands  continue  to  differentiate,  the 
chromatin  is  seen  to  form  two  parallel  streaks 
in  the  convoluted  linin  bands,  but  this  duplex 
appearance  becomes  temporarily  obscured, 
though  not  obliterated,  at  a  somewhat  later 
stage.  Each  one  of  these  duplex  chromatin- 
containing  linin  bodies  is  ^chromosome  (Fig.  26). 
When  fully  formed,  the  chromosomes  assume 
the  form  of  rods,  hooks,  etc.  The  most  strik- 
ing point  about  the  chromosomes  lies  in 
the  fact  that  their  number  is  normally  quite 
constant  for  a  particular  species  of  plant. 

The  chromosomes  become  clustered  in  a 
very  characteristic  position,  and  form  a  zone 
or  plate  across  the  centre  of  the  nucleus ;  but 
preceding  this  arrangement,  and  intimately 
connected  with  it,  a  remarkable  spindle-shaped 
structure  arises  in  the  extra-nuclear  proto- 
plasm (cytoplasm).  It  is  made  up  of  fibres 
which  are  ultimately  arranged  in  very  much 
the  same  curves  as  iron  filings  take  up  when 
scattered  on  a  piece  of  paper  under  which  lie 
the  poles  of  a  horseshoe  magnet.  The  spindle- 
like  structure  extends  across  the  space  origin- 
ally occupied  by  the  nucleus,  while  the  wall 
of  the  latter  usually  (but  not  always)  dis- 
appears, and  the  only  nuclear  structure  that 


Premeioh'c 
P.M. 


3 


Meiosis 


r 

M, 


Ma 


4, 


Fig.  26. — Diagram  to  explain  the  course  of  an  ordinary  nuclear 
division  and  the  relation  to  it  of  the  meiotic  divisions. 

The  series  1-6  P  M  represents  selected  stages,  in  the  order  in  which  they 
occur  naturally,  of  an  ordinary  nuclear  division  before  reduction  in  the 
number  of  the  chromosomes  has  occurred. 

The  series  li-6i  M  represents  the  stages  which  roughly  correspond  to 
the  premeiotic  in  the  first  meiotic  (reduction)  division.  Stages  1  and  2  are 
practically  identical.  In  5  the  longitudinal  fission  previously  seen  in  2. 
is  again  clearly  visible. 

The  series  82-62  (Mg)  represents  the  second  meiotic  division.  It  shows 
the  subsequent  division  of  nucleus  A  (Mi-6i)  in  the  series  immediately 
preceding  Ma.  The  corresponding  nucleus,  B,  in  series  MI  6,  is  not  shown, 
but  its  further  division  is  exactly  like  that  of  A. 

When  A  (or  B)  in  M!  6  proceeds  to  divide,  it  does  so  without  going 
through  the  stages  1-2,  but  passes  at  once  into  A32.  The  sides  of  each 
loop  represent  the  early  longitudinal  fission  of  the  previous  series  now  be- 
coming effective.  At  42  the  halves  are  seen  in  pairs  and  they  separate  in 
5g;  in  62  is  shown  the  stage  at  which  the  nuclei  AiAg  are  finally  going  back 
to  a  resting  condition. 


230  PLANT  LIFE 

persists  are  the  chromosomes.  These  bodies, 
as  already  indicated,  take  up  a  very  definite 
position,  and  now  lie  in  an  equatorial  plane 
half  way  between  the  two  poles  to  which 
the  spindle  fibres  converge. 

Each  chromosome  then  divides  longitudin- 
ally into  two  symmetrical  halves,  probably 
along  the  line  of  the  parallel  streaks  of  chro- 
matin  described  above.  The  two  halves  then 
diverge,  and  the  daughter  chromosomes  at  once 
retreat  along  the  spindle  fibres,  to  form  two 
groups,  one  at  either  pole.  The  daughter 
chromosomes  swell  up,  a  nuclear  wall  is 
formed  around  them,  some  of  the  substances 
which  escape  from  them  run  together  and 
form  new  nucleoli,  and  thus  the  two  daughter 
nuclei  are  formed.  A  cell  wall  is  often  de- 
veloped across  the  spindle  which  persists  for 
a  time  between  the  two  nuclei  which  have 
thus  been  constituted,  and  nuclear  division  is 
then  followed  by  cell  division.  When  the  cell 
wall  is  not  so  formed,  a  binucleate,  or  later 
a  multinucleate,  arrangement  is  produced. 

The  main  conclusions  that  emerge  from  a 
consideration  of  the  facts  thus  briefly  out- 
lined are  :  (1)  The  number  of  the  chromo- 
somes is  constant,  and  individual  peculiari- 
ties in  form  and  size  are  seen  to  reappear 
whenever  the  chromosomes  are  sufficiently 
contracted  as  to  become  sufficiently  clearly 
recognisable.  (2)  The  chromosomes,  when 
they  divide,  transmit  their  peculiarities  to 
each  daughter  chromosome.  In  other  words, 


CELL-NUCLEUS—FERTILISATION    231 

the  chromosomes  are  constant  in  qualities 
and  properties  from  one  cell  generation  to 
another.  (3)  Owing  to  the  mode  of  division, 
and  distribution  of  the  chromosomes  at  a 
nuclear  division,  the  two  daughter  nuclei  are, 
to  all  appearance,  exactly  alike,  each  is  the 
reflected  image  of  the  other.  Subsequent 
dissimilarities  in  size,  and  likewise  in  other 
respects,  are  not  excluded,  but  these  are 
almost  certainly  of  secondary  importance. 

What  does  all  this  mean  ?  We  cannot  as 
yet  give  a  complete  answer  to  the  question, 
but  a  consideration  of  the  events  connected 
with  the  differentiation  of  the  sexual  cells 
will  perhaps  serve  to  throw  some  light  on  the 
problems  involved. 

In  the  first  place,  we  have  seen  that  the 
sexual  act  consists  essentially  in  the  fusion 
of  two  nuclei.  How,  then,  can  we  reconcile 
this  with  the  circumstance  that  the  number 
of  chromosomes  is  constant  in  the  cell  nuclei  ? 
For  it  is  evident  that  the  nucleus  of  each 
fertilised  egg  must  contain  twice  as  many 
chromosomes  as  those  present  in  the  nucleus 
of  each  of  the  fusing  gametes. 

The  solution  of  this  problem  is  furnished 
by  a  most  remarkable  nuclear  division  which 
is  invariably  intercalated  somewhere  in  the 
series  of  nuclear  divisions  that  intervene 
between  the  first  formation  of  the  embryo 
at  fertilisation  and  the  final  production  of 
sexual  cells  which  closes  the  life  cycle  of  the 
organism  (Fig.  26,  l-6i). 


232  PLANT  LIFE 

In  this  particular  nuclear  division  we  find 
that  the  chromosomes  are  not  longitudinally 
divided  and  the  moieties  then  distributed  be- 
tween the  two  daughter  nuclei,  but  that  the 
whole  process  is  carried  through  in  another 
way. 

The  earliest  stages  resemble  those  of  an 
ordinary  vegetative  nucleus  which  is  about 
to  divide.  The  chromatin-containing  gela- 
tinous strands  make  their  appearance,  and  the 
chromatin  is  arranged  in  parallel  streaks. 
But  instead  of  going  on  to  differentiate  and 
finally  to  divide,  the  chromosomes  proceed 
to  unite  in  pairs.  We  have  very  strong 
grounds  for  believing  that  in  no  case  is  this 
union  a  chance  one,  but  that  a  chromosome 
descended  from  one  contributed  by  the  sperm 
unites  with  another  corresponding  to  it  but 
derived  from  the  egg.  In  other  words,  each 
pair  consists  of  a  chromosome  of  maternal 
and  a  paternal  origin. 

The  net  result,  then,  of  the  approximation 
and  union  of  the  paternally  and  maternally 
derived  chromosomes  to  form  the  respective 
pairs  is,  of  course,  a  reduction  to  one-half  of 
the  number  apparently  present. 

Each  pair  now  behaves  as  if  it  were  a  single 
chromosome.  They  flock  to  the  equator  of 
the  spindle,  but  when  they  divide  there,  what 
happens  is  simply  a  disjunction  of  the  two 
members  of  each  pair,  one  of  the  members  re- 
treates  to  one  pole,  the  other  one  to  the  other 
pole.  Hence  a  real  reduction  is  now  effected, 


CELL-NUCLEUS—FERTILISATION    233 

for  each  of  the  two  daughter  nuclei  receives 
entire  chromosomes,  but  of  course  the  original 
number,  now  shared  between  two  nuclei,  is 
really  reduced  in  each  of  them  to  one-half  of 
what  it  originally  was  in  previous  nuclear 
divisions. 

It  will  be  remembered  that  prior  to  the 
temporary  union  of  the  chromosomes  to  form 
the  pairs,  each  one  of  them  showed  indications 
of  longitudinal  fission.  It  is  of  special  interest, 
then,  to  find  that  immediately  on  the  form- 
ation of  the  daughter  nuclei,  in  the  way  just 
described,  this  fission  becomes  operative.  For 
a  second  division  supervenes  in  each  daughter 
nucleus,  and  so  four  nuclei  are  produced.  The 
reduced  number  of  chromosomes  in  each 
nucleus  is,  of  course,  maintained.  Indeed,  it 
invariably  happens  that  all  nuclei  which  are 
derived  from  one  in  which  reduction  has 
occurred  only  possess  the  halved  quantity. 
It  is  not  until  the  union  of  the  sexual  cells 
takes  place  that  the  original  number  is  again 
restored. 

The  term  meiosis  has  been  applied  to  this 
process  of  reduction,  and  meiosis  occurs  in 
every  animal  and  plant  which  reproduces 
itself  sexually  (with  possible  exceptions,  per- 
haps, in  some  of  the  lowest  and  most  aberrant 
types).  Not  only  so,  but  even  the  details  of 
the  process  are  remarkably  similar  in  the 
many  species  of  animals  and  plants  which 
have  been  studied.  ^ 

Now  it  is  hardly  possible  that  a  process  so 


234  PLANT  LIFE 

complex,  so  clearly  related  to  the  sexual  act, 
and  so  similar  in  its  details  in  the  animal  and 
vegetable  kingdoms  alike,  can  be  devoid  of  sig- 
nificance. It  emphasises  the  individuality  of 
the  chromosomes  in  the  strongest  way,  and  in 
this  respect  it  is  in  accordance  with  results  of 
many  experiments  which  indicate  that  the 
chromosomes  are,  as  a  matter  of  fact,  different 
from  one  another,  i.  e.  possess  an  individuality 
of  their  own.  Moreover,  we  see  that  in  the 
nuclei  before  meiosis,  the  chromosomes  are 
present  as  pairs  of  homologous  individuals, 
the  individuals  of  each  pair  having  originated, 
one  from  the  sperm,  the  other  from  the  egg, 
at  the  act  of  fertilisation  to  which  the  plant 
owed  its  existence.  Furthermore,  there  is 
a  considerable  body  of  evidence  to  show  that 
the  chromosomes  in  some  way  represent  the 
agents  by  which  hereditary  qualities  are  trans- 
mitted from  each  parent.  Meiosis  provides 
an  obvious  method  by  which  the  qualities, 
through  the  agents  that  are  responsible  for 
them,  may  be  shuffled  in  the  sexual  cells; 
and,  as  a  matter  of  fact,  when  hybrids  are 
inbred,  or  when  plants  are  crossed  with  one 
another  in  a  variety  of  ways,  we  find  the 
results  agree  in  practice  very  closely  with 
what  is  deduced  as  possible  from  a  study  of 
the  behaviour  of  chromosomes.  Indeed,  it  is 
not  going  too  far  to  say  that  in  meiosis  and 
fertilisation  we  are  witnessing  the  chief  act 
of  distributing  and  recombining  the  very 
substances  which  determine  the  possibilities 


CELL-NUCLEUS—FERTILISATION    235 

of  future  development  on  the  part  of  the 
offspring. 

Naturally,  the  whole  story  of  the  nucleus 
in  its  relation  to  heredity  is  a  very  long  one, 
and  in  this  brief  sketch  it  has  not  been  possible 
to  attempt  more  than  to  indicate,  in  the 
barest  outline,  a  few  of  the  most  important 
features  of  meiosis  and  of  fertilisation. 

Meiosis  has,  however,  a  further  claim  on 
our  attention,  inasmuch  as  it  has  served  as 
the  starting-point  for  some  of  the  most  strik- 
ing morphological  developments  in  the  whole 
series  of  higher  plants. 

It  has  been  seen  that  sexual  cells  cannot, 
as  a  rule,  arise  until  after  the  nuclei  have 
undergone  meiosis.  It  might,  perhaps,  be 
expected  that  immediately  the  meiotic  phase 
is  over,  the  four  cells  which  result  from  it 
would  at  once  become  sexual  gametes.  In 
animals  this  commonly  is  the  case — for  in  the 
male  animal  the  four  sperms  arise  by  the  direct 
transformation  of  the  cells  and  nuclei  that  have 
just  passed  through  meiosis.  In  the  female 
the  same  is  true,  for  the  ripe  egg,  together 
with  the  three  transitory  polar  bodies,  form 
the  corresponding  female  gametes.  Of  these, 
however,  only  the  egg  is  normally  functional. 

In  plants,  on  the  other  hand,  the  four  cells 
formed  at  meiosis  never  differentiate  directly 
into  sexual  cells — at  least  no  instance  of  their 
doing  so  is  yet  known.  Often  a  long  series 
of  cell  generations  intervenes  between  meiosis 
and  the  formation  of  gametes.  The  four  cells 


236  PLANT  LIFE 

formed  at  meiosis  often  separate  as  four 
spores,  each  of  which  may  give  rise  to  a  new 
plant  destined  in  time  to  produce  gametes. 
Thus  meiosis  in  plants  has  come  to  be  asso- 
ciated with  a  special  kind  of  reproductive 
multiplication  which  is  sometimes  called 
asexual  reproduction. 

It  would  be  better  to  replace  the  terms 
sexual  and  asexual  reproduction  by  the  terms 
gametic  and  meiotic  reproduction,  and  thus 
do  away  with  a  misleading  antithesis.  For 
"  asexual  "  and  "  sexual  "  reproduction  are 
parts  of  one  process,  carried  through  in  two 
stages.  The  two  phases  of  reproduction, 
gametic  and  meiotic,  in  all  the  higher  plants  are 
associated  with  two  distinct  stages  in  the  life 
history.  One  of  these  begins  with  the  fertilisa- 
tion of  the  egg,  and  ends  in  the  meiotic 
divisions.  The  spores,  which  are  formed  as 
the  result  of  meiosis,  inaugurate  the  second 
stage  of  the  life  history  in  which  the  differ- 
entiation of  sexual  cells  takes  place. 

This  rhythmic  alternation  of  a  spore-pro- 
ducing with  a  gamete-producing  generation 
is  well  illustrated  by  the  fern.  Starting  with 
the  fertilised  egg,  an  embryo  is  produced, 
which  grows  into  the  ordinary  fern.  If  the 
backs  of  the  leaves  are  inspected,  brown  spots 
or  stripes  may  often  be  seen,  and  these  are 
found  to  consist  of  small  capsules  or  spor- 
angia. A  young  sporangium  contains  a  fairly 
definite  mass  of  internal  cells  which  are 
enclosed  by  nutritive  tissues,  the  whole  being 


CELL-NUCLEUS—FERTILISATION    237 

encased  by  the  sporangial  wall.  The  central 
cells  increase,  and  whenever  their  nuclei 
divide,  the  full,  unreduced  number  of  chromo- 
somes can  be  seen,  just  as  it  may  be  ob- 
served in  any  other  dividing  nuclei  of  the  fern 
plant. 

But  a  time  arrives  when  the  central  cells 
within  the  sporangium  become  free  from 
each  other.  Each  one  proceeds  to  grow,  and 
it  finally  divides  twice,  to  give  rise  to  four 
spores.  It  is  during  these  two  divisions  that  the 
reduction  in  the.  number  of  chromosomes  takes 
place  in  the  manner  already  described,  and 
hence  the  nucleus  of  each  spore  only  contains 
half  the  number  of  these  nuclear  structures. 
When  the  spores  are  ripe  the  sporangium 
bursts  and  the  spores  are  scattered.  If  they 
happen  to  alight  on  a  suitable  spot,  they 
germinate,  but  they  do  not  bring  forth  a  plant 
like  a  fern  (Fig.  27).  A  filamentous  body  is 
formed  which  gradually  develops  into  a  heart- 
shaped  green  expansion  known  as  a  prothallus. 
It  is  very  delicate,  and  is  easily  dried  up,  and 
consequently  is  only  suited  to  live  where 
conditions  of  moisture  prevail. 

The  prothallus  sometimes  multiplies  vegeta- 
tively,  by  the  dying  off  of  part  of  the  plant, 
while  the  living  fragments  grow  into  new 
prothalli.  Sexual  organs,  called  antheridia 
and  archegonia,  are  developed  on  its  under 
side.  In  the  former  a  number  of  sperms  are 
produced,  while  each  archegonium,  when 
mature,  contains  a  single  egg.  We  need  not 


238 


PLANT  LIFE 


enter  into  the  details  of  their  development, 
but  we  may  note  that  each  is  a  specialised 
cell — not  only  are  the  two  gametes  different 
from  each  other,  but  they  are  different  from 


Fig.  27. — I.  Prothallus  of  fern ;  the  lower  surface,  AB,  indi- 
cates the  direction  in  which  the  section  shown  in  II  is  cut. 
II.  Section  through  an  antheridium  (An)  and  archegonium 
(Arch)  in  which  is  situated  the  egg,  E. 

all  the  other  cells  of  the  prothallus.  Each 
consists  of  a  naked  mass  of  nucleated  proto- 
plasm, but  while  the  sperm  consists  mainly 
of  nucleus,  the  egg  contains  a  very  large 
amount  of  cell  protoplasm.  We  do  not  as 
yet  know  exactly  on  what  the  definitely 


CELL-NUCLEUS—FERTILISATION    239 

sexual  nature  of  these  two  kinds  of  gametes 
depends,  but  it  seems  pretty  clear  that  it  is 
connected  with  the  shedding  off  of  some 
substance  during  the  course  of  their  develop- 
ment. 

When  a  sperm  unites  with  an  egg,  the 
life  history  enters  on  the  "  fern  "  stage.  The 
fern  plant,  like  the  prothallus,  may  undergo 
vegetative  multiplication  in  various  ways, 
but  sooner  or  later  this  "  asexual "  generation 
normally  culminates  in  the  production  of 
spores,  just  as  the  prothallial  generation 
closes  with  the  production  of  gametes. 
But  the  fern  does  not  always  go  rigidly 
through  these  stages  in  a  perfectly  invariable 
manner.  We  are  acquainted  with  a  number 
of  kinds  in  which  the  spore-bearing  fern  leaf 
may  grow  out  directly  into  a  prothallus. 
Sometimes  a  prothallus  sprouts  from  a  spor- 
angium, and  then  all  the  spores  die  away. 
Furthermore,  these  prothalli  may  bear  male 
and  female  sexual  organs,  and  from  the  egg 
a  new  fern  plant  may  arise.  What  has 
become  of  alternation  of  generations  in  such 
a  case,  and  how  are  meiosis  and  fertilisation 
respectively  affected  ? 

Taking  the  second  point  first,  it  may  at 
once  be  said  that  prothalli  formed  in  this  way 
resemble  the  fern  in  that  their  nuclei  have  not 
undergone  reduction.  Meiosis  has  been  omitted 
from  the  life  history.  But  as  a  consequence 
of  this,  the  egg  is  already  provided,  as  also 
are  the  sperms,  with  a  double  set  of  chromo- 


240  PLANT  LIFE 

somes.  It  invariably  happens,  so  far  as  at 
present  is  known,  that  when  the  eggs  are 
fertile  at  all  they  produce  new  ferns1  directly, 
that  is,  without  fertilisation.  Moreover,  even 
the  tissue  cells  of  such  a  prothallus  may 
change  their  mode  of  growth,  and  develop 
into  fern  plants  without  the  definite  produc- 
tion of  sexual  organs  at  all. 

Such  a  departure  from  the  normal  course 
of  life  history  strongly  emphasises  the  relation 
of  meiosis  to  fertilisation,  but  it  does  more 
than  this.  It  indicates  that  the  striking 
difference  between  the  fern  plant  and  the 
prothallus  is  not  itself  essentially  bound  up 
with  those  nuclear  changes  which  are  inti- 
mately associated  with  the  sexual  phases. 
It  points  rather  to  the  conclusion  that  in  these 
plants  the  life  history,  with  its  two  different 
stages,  may  have  developed  in  coincidence, 
though  not  in  causal  connection  with  the 
separation  of  the  sexual  process  into  two 
stages.  It  would  clearly  be  futile,  in  the  face 
of  the  evidence,  to  attempt  to  maintain  the 
existence  of  a  causal  relation  between  the 
nuclear  changes  and  the  characteristic  differ- 
ences between  the  two  stages  of  the  life 
history  of  the  fern.  In  this  way  we  may 
understand  the  continuance  of  the  alternate 
appearance  of  fern  and  prothallus,  even  when 
the  cellular  rhythm  no  longer  obtains. 

Considerations  of  space  preclude  the  follow- 
ing up  of  this  matter  in  any  detail;  it  may, 
however,  be  said  quite  generally  that  wherever 


CELL-NUCLEUS—FERTILISATION    241 

fertilisation  recurs  meiosis  is  never  omitted,1  and 
this  is  true  for  animals  as  well  as  for  plants. 
The  ordinary  course  of  life  histories  has  been 
developed  long  after  sexuality  and  meiosis 
appeared,  and  has  progressed  independently, 
and  on  different  lines  in  different  groups. 
Sometimes,  as  in  the  higher  plants,  the  stages 
of  the  life  history  are  more  or  less  obviously 
connected  with  these  nuclear  cardinal  points, 
at  other  times  the  relation  is  not  so  evident. 
For  example,  it  may  happen,  as  in  many  of 
the  flowering  plants,  that  the  two  stages  in 
the  life  history  so  well  separated  and  analysed 
in  the  fern,  become  curtailed.  This  happens 
when  the  cell  which  should  give  rise,  by  the 
two  divisions,  to  four  spores,  cuts  the  process 
short,  grows,  and  itself  becomes  the  spore 
without  any  division.  Such  a  short  cut  is 
taken  in  certain  of  the  sporangia  (ovules)  of 
a  lily,  orchis,  and  many  other  plants.  But 
meiosis  is  not  cut  out.  It  supervenes  at  the 
very  next  divisions  which  follow  the  omitted 
stages  during  which  it  would  normally  have 
been  effected. 

As  we  advance  to  types  of  plants  above  the 
ferns  we  find  the  life  history  becoming  more 
complicated  and  less  diagrammatically  clear. 
The  principle  which  underlies  the  complica- 
tion is,  however,  a  simple  one;  it  consists 
in  a  provision  for  giving  the  sexually  produced 
embryo  an  advantageous  start  in  life. 

1  The  occasional  anomaly  reported  for  certain  mosses 
requires  further  investigation. 

Q 


242  PLANT  LIFE 

Even  among  plants  nearly  related  to  the 
ferns  we  find  that  the  prothalli  produced  by 
the  spores  tend  to  differ  in  their  capacity  for 
growth.  Those  which  are  destined  to  produce 
eggs  are  large  and  well  stocked  with  food,  those 
which  will  produce  the  sperms  are  small. 
This  difference  becomes  reflected  in  the 
spores,  and  even  in  the  sporangia. 

In  the  Selaginella  plants,  often  grown  in 
greenhouses,  certain  of  the  sporangia  produce 
a  large  number  of  small  spores,  whilst  others 
become  much  larger,  but  only  bring  a  very 
few  (usually  four)  large  spores  to  maturity. 

The  small  spores  form  a  rudimentary 
prothallus  and  a  larger  or  smaller  number  of 
sperms.  It  is  essential  that  the  number  of 
small  spores  should  be  kept  up,  so  as  to 
maintain  a  fair  chance  of  a  sperm  from  one  of 
them  reaching  the  relatively  few  available 
female  prothalli. 

Passing  to  the  flowering  plants,  it  is  difficult 
at  first  sight  to  realise  that  we  are  only 
witnessing  the  final  stages  of  an  evolutionary 
development  of  the  structures  so  clearly 
distinguishable  in  the  fern.  But  so  it  is,  and 
it  will  be  of  interest  to  trace,  even  briefly,  and 
in  spite  of  the  wide  gaps,  the  points  of  resem- 
blance between  them. 

The  obvious  starting-point  in  both  cases  is 
the  fertilised  egg.  The  fern  plant  and'  the 
flowering  plant  each  spring  from  this  source. 
The  fern  closes  its  life  cycle  by  producing 
spores  in  the  way  we  have  seen.  The  flowering 


ma 


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Fig.  28.— Longitudinal  section  (slightly  diagrammatic) 
through  the  pistil  of  buckwheat  (Polygonum). 


il 


244  PLANT  LIFE 

plant  does  the  same,  but  the  subsidiary 
events  that  have  happened  during  its  evolu- 
tion obscure  its  record.  However,  we  find 
two  kinds  of  sporangia,  one  commonly 
grouped  in  a  cluster  of  two  or  four,  and  form- 
ing the  so-called  pollen  sacs  borne  on  each 
of  the  stamens.  The  pollen  sacs  produce  the 
spores  or  pollen  grains  in  much  the  same 
way  as  the  spores  are  formed  in  a  fern.  But 
the  other  sort  of  sporangium  is  less  easily 
recognised  (Fig.  28).  It  is  often  known  as  an 
ovule,  and  the  ovules  are  situated  inside  a 
closed  cavity  called  the  ovary,  which  forms 
the  lower  part  of  the  pistil  of  the  flower. 
Each  ovule  (or  sporangium)  usually  contains 
but  one  spore,  and  this  is  not,  like  the  pollen 
grain,  thrown  out  of  the  sporangium,  but 
germinates  inside  it,  and  produces  an  egg  as 
well  as  a  number  of  other  cells.  Moreover, 
the  sporangium  is  retained  within  the  ovary, 
and  hence  the  methods  of  fertilisation  which 
are  appropriate  for  a  fern  would  clearly  be 
impossible  here.  As  a  matter  of  fact,  the 
pollen  grain,  i.  e.  the  spore  from  which  the 
male  gamete  will  be  derived,  has  to  be  brought 
into  special  relation  with  that  part  of  the 
flower  in  which  the  ovule  is  situated.  At  the 
summit  of  the  ovary  there  is  a  specialised 
structure  called  the  stigma.  This  is  often 
viscid  just  when  the  ovules  are  mature,  and 
thus  any  pollen  which  falls  on  to  the  stigma 
is  retained. 

The  pollen  grain  or  spore  already  contains 


CELL-NUCLEUS—FERTILISATION    245 

developed  within  it  a  very  rudimentary 
structure  which  it  is  possible  to  trace  back 
to  an  extremely  reduced  prothallus.  The 
pollen  grain  presently  begins  to  put  forth  a 
tube  which  grows  into  the  tissue  of  the  stigma, 
feeding,  like  a  fungal  hypha,  on  the  juices  it 
contains.  The  tube  grows  down  through  the 
intervening  tissue  of  the  style  into  the  cavity 
of  the  ovary.  When  it  reaches  this  it  is 
attracted  to  the  tips  of  the  ovules,  and  enters 
one  of  them  by  way  of  a  little  pore  (the 
micropyle),  burrowing  through  an  intervening 
tissue  of  the  sporangial  (i.  e.  ovular)  wall  that 
may  be  present,  until  it  reaches  the  spore. 

Meanwhile,  from  the  body  of  the  pollen 
grain  the  essential  structures  above  alluded 
to  have  entered  into  the  tube.  Two  sperms 
are  developed,  but  they  are  not  provided 
with  locomotory  cilia.  They  are  finally  dis- 
charged into  the  cavity  of  the  spore,  when 
they  at  once  lose  all  cytoplasmic  invest- 
ment, and  appear  as  naked  nuclei,  somewhat 
vermiform  in  appearance.  They  pass  through 
the  protoplasm,  which  is  contained  in  the 
spore  (or  embryo-sac  as  it  is  often  called), 
apparently  by  autonomous  movement,  and 
one  of  them  approaches,  and  finally  fuses  with, 
the  egg.  The  other  one  fuses  with  a  remark- 
able pair  of  nuclei  which  are  found  near  the 
centre  of  the  egg,  and  the  nucleus  resulting 
from  the  latter  fusion  is  responsible  for  the 
production  of  the  nutritive  matter  that 
later  on  fills  so  many  seeds  and  grains  (e.  g. 


246  PLANT  LIFE 

wheat)  with  "  albumen  "  or  endosperm.  It 
is  a  very  remarkable  fact,  this  second  fusion. 
The  sperm  nucleus  which  takes  part  in  it  is 
the  sister  nucleus  of  that  sperm  which  fuses 
with  the  egg.  Hence  it  might  be  expected 
that  it  would  carry  paternal  characters,  and 
that  these  might  make  themselves  felt  in 
the  nature  of  the  endosperm  to  which  the 
triplicate  nucleus  gives  rise.  It  turns  out 
that  the  expectation  is  realised,  and  where 
the  endosperms  of  the  pollen  parent  and  the 
seed  parent  differ  in  a  well-defined  character, 
e.  g.  in  colour  or  sugar  contents,  the  char- 
acter imported  by  the  sperm  from  the  pollen 
may  dominate  the  whole  endosperm.  Thus, 
when  pollen  grains  of  different  varieties  of 
Indian  Corn  are  blown  on  to  a  female  ear, 
the  endosperm  of  some  of  the  grains  will  be 
found  to  be  affected  by  the  characters  borne 
by  the  strange  pollen.  And  it  is  just  these 
identical  grains  that  will  betray  evidence  of 
hybrid  characters  in  the  embryo  which  each 
of  them  contains.  For  the  same  pollen  grain 
provided  both  the  sperm  for  fertilising  the 
egg,  and  also  the  second  sperm  which  formed 
part  of  the  triplicate  combination  from  which 
the  endosperm  originates. 

When  fertilisation  has  been  accomplished, 
remarkable  changes  are  produced,  not  only 
within  the  ovule,  but  outside  it  as  well. 
Within,  the  endosperm  arises,  by  the  re- 
peated division  of  the  triplicate  nucleus  as 
already  explained,  while  at  the  upper  end 


CELL-NUCLEUS— FERTILISATION    247 

of  the  ovule  the  embryo  begins  to  develop. 
Gradually  the  ovule  changes  into  the  seed. 
Reserve  materials  of  food  accumulate  within 
it,  and  are  most  frequently  stored  either 
in  the  growing  endosperm,  or  partly  (seldom 
wholly)  in  the  sporangium  wall  (nucellus, 
perisperm).  If  the  embryo  reaches  any  con- 
siderable size  within  the  seed,  it  may  presently 
destroy  these  tissues,  and  absorb  the  nutritive 
contents  into  its  own  body.  When  this 
happens,  some  part  of  the  young  plantlet 
usually  becomes  thickened  and  so  forms  the 
repository  for  the  food.  Most  commonly  it 
is  the  seed  leaves  (cotyledons),  as  in  the 
bean,  or  it  may  be  the  young  stem  below 
them,  as  in  the  brazil-nut,  which  thus  becomes 
charged  with  the  reserves  of  food. 

In  whatever  way  the  food  material  is  dis- 
posed, however,  it  is  always  so  situated  as  to 
be  readily  available  when  the  young  plantlet 
starts  into  growth,  on  the  germination  of  the 
seed. 

It  does  not  invariably  happen  that  con- 
siderable stores  of  food  after  this  fashion 
await  the  embryo  on  its  awakening  to  its 
new  life.  Many  of  the  flowering  plants  have 
followed  other  lines  than  that  of  transmitting 
to  a  relatively  small  posterity  large  accumula- 
tions of  hereditary  capital.  The  commonest 
alternative  is  seen  in  the  production  of  vast 
quantities  of  small  seeds.  The  seed  and  the 
contained  embryo  have  been  well  cared  for 
during  the  earlier  stages — but  they  are  cast 


248  PLANT  LIFE 

out  from  the  parent  plant  with  the  scantiest 
supplies  of  ready-made  nutriment.  Hence, 
on  germination,  they  must  quickly  begin  to 
make  their  own  living. 

Both  methods  have  proved  successful  in 
different  lines.  The  advantage  of  small  seeds 
lies  in  the  number  of  offspring  produced, 
and  in  the  ease  with  which  their  dispersal  is 
ensured.  Of  course,  it  is  inevitably  accom- 
panied by  great  mortality — a  waste  in  so  far 
as  the  individuals  are  concerned,  but  by  no 
means  necessarily  so  from  the  point  of  view 
of  the  race. 

Parasites  generally  (though  not  invariably) 
produce  huge  quantities  of  small  seeds.  The 
profitable  result  is  sufficiently  obvious,  for 
the  individual  chances  of  success  cannot,  at 
best,  be  very  great — a  species  that  relied  on  few 
seeds  would,  in  the  majority  of  cases,  be  placed 
at  a  disadvantage,  inasmuch  as  the  conditions 
of  successful  development  can  only  be  seldom 
realised.  Every  unsuccessful  individual  would 
naturally  be  exterminated,  and  thus,  with  a 
scanty  progeny  the  race  itself  might  easily 
die  out.  Moreover,  the  advantage  of  big 
seeds  is  less  in  the  case  of  a  parasite  than  in 
that  of  ordinary  plants,  because  if  a  seed 
secures  a  lodgment  enabling  the  embryo  to 
attack  a  suitable  host,  nutrition  in  abundance 
is  ready  to  hand.  But  for  those  that  fail 
to  reach  a  host,  no  stock  of  nutrition,  however 
great,  would  be  of  any  real  avail. 

It  matters  little  in  what  direction  we  cast 


CELL-NUCLEUS—FERTILISATION    249 

our  attention  on  the  manifold  variety  ex- 
hibited by  plants,  the  adaptedness  of  species 
to  their  environment  is  always  one  of  the 
most  striking  of  their  many  qualities.  But, 
as  we  have  seen,  this  adaptedness  is  intrinsic- 
ally the  result  of  the  inner  constitution  of 
the  plant,  which  impels  it  of  necessity  to 
develop  in  this  or  that  particular  manner. 
Only  those  plants  whose  constitutions  are  such 
as  to  cause  their  development  to  be  adapted 
to  a  given  environment  can  flourish  under  the 
particular  conditions  imposed  by  it. 

Adaptedness  is  often  achieved  in  an  indirect 
fashion,  but  it  must  be  susceptible  of  realisa- 
tion in  some  way  or  another  if  the  individual 
is  to  survive. 

Every  species,  just  as  every  individual  of 
the  species,  has  to  face  its  critical  problems. 
And  the  problems  of  the  species  are  really 
the  same,  though  sometimes  disguised  under 
different  forms,  as  those  which  confront  the 
individual.  The  race  problems  are  solved  by 
the  individuals,  often  in  a  wonderful  way. 
Thus  many  tolerably  heavy  fruits  are  dis- 
persed by  a  wing-like  outgrowth  which  delays 
their  descent  to  the  ground.  But  at  an 
earlier  stage  this  wing-like  outgrowth  is 
generally  green,  and  so  may  well  have  helped 
in  the  nutritive  processes.  We  can  state  with 
confidence  that  it  was  not  developed  in  order 
to  aid  in  the  dispersal  of  the  fruit,  but  that  it 
arose  as  the  result  of  far  backward-reaching 
correlations  of  ultimate  structure  and  chemical 


250  PLANT  LIFE 

processes  within  the  parent  organism.  Inci- 
dentally, however,  as  the  wing-like  structure 
dries  up,  its  adaptedness  for  assisting  dispersal 
may  become  of  inestimable  value  to  the 
species. 

The  individual  has  its  duty  to  itself,  and 
its  own  immediate  success  is  measured  by 
the  completeness  with  which  it  is  adapted 
to  overcome  the  difficulties  that  beset  it — 
difficulties  that  arise  partly  from  within,  and 
partly  also  from  without — and  so  to  emerge 
victorious  in  the  struggle  for  existence.  But 
unless  it  is  so  constructed  as  to  launch  a 
successful  posterity  on  the  world,  its  race  is 
destined  in  the  long  run  to  perish,  and  "  The 
place  thereof  shall  know  it  no  more." 


BIBLIOGRAPHY 


GENERAL  WORKS 

GOEBEL,  K.     The  Organography  of  Plants     Translated  from 
the  German    by    I.   Bayley  Balfour     (Clarendon    Press, 
Oxford). 
Contains  an  admirable  account  of  the  organisation  of  plants. 

KEENER  v.  MARILAUN,  A.  The  Natural  History  of  Plants. 
Translated  from  the  German  by  F.  W.  Oliver  (Henry 
Holt  &  Co.,  1894). 

A  fine  work  in  two  large  volumes,  dealing  with  many  aspects 
of  plant  life,  but  with  a  strong  teleological  bias. 

SCHIMPER,  A.  F.  W.    Plant  Geography.    Translated  from  the 
German  by  P.  Groom  and  I.  Bayley  Balfour  (Clarendon 
Press,  Oxford). 
A  comprehensive  work  dealing  with  the  distribution  of  plants 

over  the  world,  with  many  illuminating  details  of  adaptive 

modifications. 

TANSLEY,    A.   S.     Types    of  British    Vegetation    (Cambridge 

University  Press). 
Deals  especially  with  the  ecology  of  plants. 

TIMIRIAZEFF,  0.  A.     The  Life  of  the  Plant.    Translated  from 
the  Russian  by  Miss  Che're'me'teff  (Longmans,  Green  &  Co.). 
Treats  the  plant  from  a  physiological  point  of  view. 

THE  SOIL 

HALL,  A.  D.     The  Soil  (John  Murray). 
An  excellent  treatise  on  the  soil  in  its  bearing  on  vegetation. 

RUSSELL,  E.  J.     Lessons  on  Soil. 

An  elementary  and  good  practical  introduction  to  an  under- 
standing of  the  soil  and  its  problems. 
251 


252  BIBLIOGRAPHY 

HEREDITY 

BATESON,  W.  Mendel's  Principles  of  Heredity  (Cambridge 
University  Press). 

DE  VRIES,  H.  The  Mutation  Theory.  Translated  from  the 
German  by  J.  B.  Farmer  and  A.  D.  Darbishire  (The  Open 
Court  Publishing  Co.). 

THOMSON,  J.  A.    Heredity  (John  Murray,  London). 

Smaller  works  dealing  with  various  aspects  of  plant  life  are — 
CZAPEK,  F.     The  Chemical  Phenomena  of  Life  (Harpers). 

LOEB,  J.  The  Mechanistic  Conception  of  Life  (University  of 
Chicago  Press). 

SCOTT,  D.  H.     Structural  Botany  (A.  &  C.  Black). 

WARD,  H.  M.  Diseases  of  Plants  (Society  for  the  Promotion 
of  Christian  Knowledge). 

WEST,  G.  S.  The  British  Freshwater  Algce  (Cambridge 
University  Press.). 


INDEX 


ABSORPTION  of  water  from 

the  soil,  79 
Adaptation       in       climbing 

plants,  117 
Adaptation   in    plants,    127, 

248 
Alternation    of    generations, 

236 

Apiocystis,  33 
Asexual  reproduction,  236 

Bacillus     of     root-tubercles, 

195 
Bauhinia,    a    climber,     114, 

116 
Begonias,     propagation     of, 

210 

Bird's-nest  orchis,  190 
Bleeding  of  trees,  84 
Bordered  pits,  78 
Broomrape,     parasitism     of, 

186 

Bud-scales,  135,  142 
Bulbils  of  ferns,  209 
Bulbous  plants,  144 

Cacti,  142 
Cambium,  100 
Caulerpa,  50 
Cell,  20 
Cellulose,  14 
Chlamydomonas,  15 
Chlorophyll,  20 


Chloroplast,  17 
Chromatin,  227 
Chromosome,  228 
C.lia,  13 
Cladophora,  44 
Climbing  plants,  107 
Collenchyma,  97,  98 
Co-ordination,  42 
Cuticle,  62 

Diffusion    of   gases    through 

stomata,  65 

Dischidia  rafflesiana,  159 
Dodder,  188 

Endosperm,  245 

Epichloe  typhina,  attack  by, 

179 

Epiphyte,  133,  149 
Epiphytic  orchids,  151 
Epiphytic  Tillandsias,  155 

Fertilisation    in    Chlamydo- 
monas, 214 

Fertilisation  in  Halidrys,  223 
Fertilisation  in  Fucus,  221 
Flowering  plants,   reproduc- 
tion in,  242 
Fluorescence  of  chlorophyll, 

24 
Fungi,  161 


Gamete,  213 


253 


254 


INDEX 


Gemmae      as        propagative 

bodies,  208 
Grass   stems,   mechanics   of, 

98 

Hsematococcus,  26 
Halimeda,  61 
Hydrurus,  40 
Hygrophyte,  138 

Immunity,  177 

Laminaria,  48 

Leaf-fall,  136 

Leaf,  function  of,  61 

Leaves,  mechanical  tissues 
of,  123 

Leaves,  physiological  replace- 
ment of,  139 

Lichens,  199 

Lithophyte,  133 

Loranthus,  a  flowering  para- 
site, 184 

Mangrove  plants,  120 

Mechanical  tissues,  90 

Mechanical  tissues  of  climb- 
ing plants,  109 

Mechanical  tissues  of  roots, 
103 

Meiosis,  233 

Mesophyte,  138 

Mistletoe,  182 

Mycorhiza,  192 

Nuclear  division,  227 
Nucleus  of  the  cell,  17,  226 

Organisation,  58 
Ovule    of    flowering    plants, 
244 

Parasites,  165 
Parasitism,  stages  in,  185 


Peltigera  canina,  201 

Photosynthesis,  23 

Physiological  drought,  133 

Pleurococcus,  19 

Pollen,  244 

Prasiola,  38 

Prothallus  of  fern,  226,  237 

Rafflesia  Arnoldii,  a  flowering 

parasite,  187 
Reproduction,  204 
Root  pressure,  85 
Root,  structure  of,  72 
Root    tubercles    of    Legumi- 

nosae,  194 
Roots  assuming  function  of 

leaves,  140 
Roots,  mechanical  tissues  of, 

103 
Rust-fungi,  180 

Saprophytes,  165 
Sclerenchyma,  94 
Sea-lettuce,  36 
Sexual  reproduction,  212 
Smut  infesting  campion,  173 
Soil,    absorption    of    water 

from  the,  79 

Spectrum  of   chlorophyll,  25 
Spirogyra,  43 
Sporangia,  236 
Starch  in  leaves,  67,  69 
Stoma,  64 

Stress,  effects  of,  129 
Symbiosis*  of  lichens,  200 
Syncytium,  21 

Thickening  of  stems,  101 
Tillandsia  usneoides,  155 
Timber  trees,  growth  of,  87 
Tracheids,  76 
Transpiration,  70,  136 

Ulva,  36 


INDEX  255 

Vascular  bundle,  62  Witches'-brooms,  174 

Wood,     destruction     of     by 

Water   content   of   the  soil,  fungi,  169 

81 

Water,  importance  of,  29  Xerophyte,  138 
Water,     movement     of     in 

plants,  83  Zoospores,  35,  207 

Water  plants,  120  2ygote,  213 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


JAN  29  1526 
FCB   8  192C 

^  :. 


JUN 


12  1941 


'.6 


30m-6,'14 


JRIOLOG  t 


268794 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


